Beasts of Chaos
The beastmen are not an easy army to play. You will find that your book is underpowered and that a lot of unfavourable rulings are made against you. Having said that I love the beastmen.
Fast moving infantry
Access to monsters
No ranged weapons.
Beastmen are the guerilla force of WHFB. A beastmen army should rely on it's mobility to put several units against a enemy unit to ensure victory. A BoC army should play like a wolfpack hunting it's foes. You will depend on combo charges as a beastman unit ain't spectacular. Low leadership will also hammer home this point of redunency particurally when fighting the undead or daemons. A good beast general plans for his units to fail.
The type of army you cna field is varied with the Boc. You can go for the standard force led by a beastlord which is depent on it's beastherds and chariots to get the job done and use monsters in support. Or you can play a force led by a doombull which allows you to take minos as core. This type of army is similar to the Ogre kingdoms list as it relies on fear and fast mobile monsters. Also you must choose what mark to take. Undivided is for those who can't decide and allows you the most choice of other marked units.
Khorne gives you frenzy and dispell dice. This mark is as good as the player using it. Against someone who isn't prepared you will dominate. If someone however is able to use frenzy aghainst you, you'll be lead all over the board. A personal favourite of mine I rate this mark as the best as it also protects the beastmen from their low Leadership.
Tzeentch turns characters into casters and units give Power dice. The best mark for characters and the worse for units it allows you to pump out a barrage of magicial attakc s but severlly lessens your units Lds protections and gives them no benefits in the game. If you like magic then I recommend this, remember the BoC lore of tzeetch isn't the best lore in the game though.
Nurgle is a funny mark. It gives your units fear, and scaly skin to minos whcih is great. Previously I found it lacking but in the current metagame (fear causing enemies common) it becomes alot better. If you want a mino force then it's the best mark otherwise meh. The lore is primarly more support.
Slaanesh is IMHO the worse mark. It give your unit a immunity to psychology whcih for a army like the Boc who depend on being able to flee is death. The LD protection is alright thoguh. However it has by far the best lore whcih can easily dominate the game. IF you want a magic heavy force it's wonderful otherwise don't bother.
The standard general for a beast army. The beastlord is best used as a super beastmen. His high movement and toughness means he is quite a good investment for a beastherd. I recommend you keep him mimally tooled to do his job and just leave him to it. Treat him like any generic leader.
Chaos armour, great weapon, shield, pelt of the dark young.
This is my favourite beastlord to run. He's cheap enough that you don't break the bank in taking him and resonably versatile, the great weapon is truly a useful weapon as his strength of 7 with it means that chariots can be popped. Pelt of dark young is there to add to your army's magicial protection .
Other variations include
Chaos armour, greatweapon, shield, .
This is a beastlord odesigned to simply be skilled at fighting things. Pelt of the dark young isn't needed if your playing a khorne army. The mark of khorne doesn't alter his proffession, which is to say, hitting things but instead makes him a more potent fighter. Be wary though he isn't able to fight other races generals if they're fully tooled up so should be treated with care.