A lot of players are attracted by the first furry warband published for Mordheim, the Skaven. If that is because they come in the starter box, because they have they special way of playing, or just because they are considered powerful. I do not know.
There are some things that immediately jump out, high stats (Move and Initiative), warband maximum of 20 characters etc.
Lets see what we can say about the starting warband. What is possible, what can characters do...

Heroes

The Assassin adept: This is a powerful hero for the warband. Special mentioning is the BS 4, which is often overlooked. And a sling is cheap and quite powerful is his hands. The WS4/S4 also makes him ideal for hand to hand fighting. Not to mention the extra -1 on armour saves he confers through the Perfect Killer skill. Even with missile weapons.
The Black Skaven: These are you main fighting machines; give them something that lets them utilize the S4 en WS4.

Nightrunners: The nice fact us that they have a BS3, which gives them a nice bonus compared to a normal young blood. Do not underestimate their fighting ability, because they advance quickly, they become dangerous quickly. The low cost makes them real skavenlike and easy to replace.

Eshin Sorcerer: Almost I would write the obligatory spell caster, but do not be mistaken, he can be used in the role of artillery support or become a combat monster.
Verminkin: These are very useful en cheap henchmen. A fully equipped henchman cost 25 GC and has a sling for ranged attacks and 2 close combat attacks (club and dagger).
Giant rats The "cannon fodder" of this warband. Do not underestimate the power of these henchmen, there are not much warbands that can buy an attack for 15 GC. And as we al know, the attack is the most powerful ability in Mordheim.
Ratogre He is the heavy hitter of the warband: T5, S5 and 3 attacks to boot. The only problem is that if he goes out of action, you have one-third chance you loose 210 GC...

Startingtips

So how would you begin with a warband, what is important to look at when you start?

1. If possible buy as much heroes as possible, but at least go for 5. They are the guys that actually bring in the money, after a battle. The money… I cannot stress that enough J
In one time only games that does not matter, so then there is more room for experimenting.
2. Slings.
They are cheap and effective. Especially if one of the nightrunners gets Eagle Eyes. Or that henchmen group gets that BS4 advance. Better give out as much as possible.
3. Daggers.
Do not worry if little money is left for weapons, let alone exotic ones (weeping blades etc.). Most skaven fight effectively with 2 daggers. Especially if they have S4 (negates the armour bonus opponents get).
For henchmen it can look good to buy a club instead of a second dagger, it costs only 1 GC more. But be sure you do not loose out on the right number of skaven, i.e. highest possible routetestnumber.
4. Giant Rats. Do not underestimate the effectiveness of a "walking attack" They maybe do not look as powerful as a wardog, but they have M6 and I4 and only cost 15 GC. They are easy to replace. Just try a couple, they'll multiply by themselves.J

Warbandtypes

So now you know something about the characters and about important starter points. Now lets get to the warband designing.
Of course there are a lot of options, but often they come down to three types of warbands.

1. (I call it:) The fluff-warband.
This warband maximizes on numbers, skaven are plenty you know. But it also uses some "nice-nice" skaven items. Lets start with an example:
Adept with WB's and sling.
2 Black skaven with sword and sling.
Sorcerer with sling and spear (this might vary, because of which spell he gets, but a club could work too. Advice on sorcerers and spells will follow).
2 nightrunners with sling and club (S&C).
3 groups of henchmen with S&C (total of 7 figures, spread out how you like).
This gives you 13 figures, thus routing starts only after loosing 4 characters. As the Weeping Blades are 50 GC, Fightingclaws, Warplocks, or a set of flails could replace this. This can be filled in, as you like. Especially the sorcerer could be in need of tuning, due to the spell he gets.

2. The maxed out warband.
Here you go for numbers and nothing else. Lets get on with another example:
This time about the only extra things your characters buy is slings.
Full heroes with slings. (max income!)
12 henchmen of which 11 have a sling and one not (499 pts).
This gives you 18 characters a route test after loosing five... You could loose one skaven verminkin and buy a sword, 2 clubs and a couple of extra daggers, then you still route at 5. Or go for the option to replace 3 verminkin by Giant Rats (5) and loose a couple of slings. Then you could start with 20.
If the sorcerer then gets his extra rats spell, you might even route only after loosing 6...
There is a possibility to start with 20 members and al with slings, but then you only start with 4 heroes: AA, BS, 2 NR, 16 Verminkin, al with slings. (Tanx rinku.)

3. The "risky" warband.
This warband can be cruelly effective at the start of a campaign, but also it can be crippled after only one battle... Example:
6 heroes with slings, al S4 guys with extra daggers
The sorcerer a spear (or use the 10 GC to upgrade the giant rats to verminkin).
The nightrunners get clubs.
A Ratogre.
2 Giant Rats.
A Ratogre is a powerful character, especially in a beginning warband. It also is very expensive (210 GC). You noticed that there are 9 characters in the warband. This is to get that 3-persons-lost-routetest.
3.1. Advantage of a Ratogre
Just look at that monster: S5, T5 and 3 attacks, what do you want more? Oh... 3 wounds to boot of course. I saw this guy rip through 3 Dark Elves like the were not there... In starting campaign he rocks. Almost nothing can hurt him really, at least not able of taking it out in a single round. Often it just forces the route test on the opponent faster. If you keep it near a hero, it does not even need to roll for passing its stupidity.
3.2. Disadvantage of a Ratogre.
It costs 210 GC and if it is taken OOA, there is a one-third chance you loose it. You have to keep it near a hero, which makes it a bit less effective. You noticed it was also an advantage, but because it links 2 characters together it slightly hampers their movement. If you buy it later in a campaign, the effectiveness is far less, because more heroes are actually capable of crippling your Ratogre, because they get those nice advances.

Magic of the Horned Rat:

Now only a last section is required for completing a starting warband. Those are the spells for the Eshin Sorcerer. He can start with one (Uf Kurz, Neek!), which you then can abuse to your harts desire.
The equipment for the Eshin Sorcerer may vary very much as to which spell he gets. Lets see. Basic... a sling. :-) Other additions are noted according to spells:

1. Warpfire. This is the main damage spell of the skaven. At least a lot of people think so (my favorite is another spell :-). Your sorcerer does not need any extra equipment bar some missile weapon and maybe some melee weapon.
The devious thing about this spell is the range of 8". This means you are in charge range of all your opponents except dwarfs. Think about that and protect him. Remember this spells does not guarantee to knock out half the opponent's warband... Equipment: Hand weapon, sling.
Example: 3 enemies close enough to utilise the spell well. The main target gets genrally 2 S4 hits, going at T3 this means about 40% chance he goes OOA. And about 25% to take out one of the "onlookers". This is hardly half a warband… next turn your sorcerer probably will get charged, or shot at…

2. Children of the Horned rat. This one is a nice one; it about guarantees you extra cannon fodder and may be used to boost rout test numbers. You get at least a boost of one.
You sorcerer is spellless after the battle starts, if he only has this one, so extra combat power is not unwise. Maybe now a spear, or a warplock would be nice, if affordable. Equipment: sling, spear.

3. Gnawdoom. About the same comment as Warpfire. Although I give this spell a better chance of wounding, but that is only based on personal experience... I like to throw a lot of dice... :-) Advantage to warpfire is that it needs not be the first model in the path, but you can choose the tartget. And normally I score 3-4 wounds…Equipment: Hand weapon, sling.
Example: Nominal you hit 7 times and you need sixes to hit, up to T4 that is. On a T3/4 character this is 1 or 2 wounds normally. Which gives you about 40% chance to take him OOA.

4. Black Fury (If there ever was a close combat spell it would be this one). The others are nifty, but not as powerful (this is my favorite spell, for those who did not get it yet... :-). The 2 extra attacks and extra +1S are really nice, but this needs weapons that work well with the advantages. Several S4 hits are nothing to sneeze at (nor is the 22" charge range), but sometimes this dude needs to defend himself. And armour is not that effective, as it does not go well with the spell casting.
So there needs to be something that replaces an armour save, like a parry, maybe even with reroll. Very effective (At least in my experience). Sprint, a +1S and Dodge are also very handy, but that is not yet in order. My beginning sorcerer sliced up a rat ogre with his 4 attacks, although I give you he already had lost one of his three wounds... Next follow the weapon choices that are interesting for an Eshin Sorcerer with Black Fury. Equipment:
Flail together with the spell this makes for 3 S6 attacks, very hurtful.
Which means hitting on 4+ wounding on 2+ and often no parry from the opponent
Weeping blades (the most expensive form) gives 4 S4 attacks, with auto wounds on 6 for to hit.

Here you have a nice bonus autowound on 6, but also a protective parry from the blades.
·
My favorite is Fighting Claws, not only for the 4 S4 attacks, but also because of the parry with reroll. This gives your sorcerer that small survivor ability he needs. Apart from the parry with reroll, (aint't it a bummer if you just fail to roll that 4+ you need, remember you sorcerer is BS3. So many heroes tend to hit him already on a 3+) it has a possible future bonus of the skill the Art of Silent Death, which only works with bare hands and Fighting Claws.
Example: basic sorcerer with FCs: in geral hits 2 with S4 so 50-60% chance of wounding and 40% chance of taking an opponent out of action, even 2 wound characters.

5. Eye o'the warp. Which is actually a nice spell too, even more if you are allowed to hit the fleeing opponents. But to make this effective you need some nice Close combat weapons, but club and dagger could work. Equipment: club/sword/spear.
Example: fighting two opponnets in HtH before you cast which means 50% chance one is already hampered, or maybe even OOA. Which makes this skill as a free combat master skill with a dual bonus. They may flee and the hits are resolved before HtH.

6. Sorcerer's curse. This one has a 12" range, which gives your sorcerer a lot of charge protection, so no extra need for special equipment, a sling will do. This will aid your skaven greatly in battle. It is more devastating as it looks. Because it can effectively hamper a single character in combat and make it easy prey for your skaven brothers. Just asked a certain warband leader (I will not name Gerhardt of course... :-), whose high number of attacks were almost rendered useless by a successful casting...

Example: this can effectively half the cjhance that a character hits his opponent, or make his armoursave… very nice against those strong heroes (statwise or skill wise).