Make sure that you always include a Battle Standard.
Wight Lords are good cheap hero level characters.
The Vampire Count is very useful due to his special abilities, excellent fighting profile, and second level magic, he is a bargain at 200 points.
Undead Champions: Make sure every unit has one of these.
Make sure to give all units a standard bearer, and use magic standards where possible.
Skeletons and Skeleton Cavalry make good light troops, but make sure you remember they are light troops, for hitting flanks and screening your more powerful troops. Skeleton Cavalry make excellent flankers, even with their low WS, as no terrain penalties apply and they cannot be broken.
Zombies are only useful in large hordes, these are about the weakest troops there is.
Ghosts, even with their low WS, can take out much better troops due to their immunity to normal weapons.
Ghouls: Always use in large blocks, with at least 4 ranks. Ghouls, due to their two attacks, even with low WS, have a chance to inflict damage to enemy units when used in deep formations, and their high Toughness gives them good survivablity, they can even tie down elite units.
Wights, with their Wight Blades, Toughness 4, and good Leadership, make good shock troops, but always put a hero in the front line (such as a Wight Lord).
Mummies have a Strength 4, Toughness 5, 4 Wounds, and 2 Attacks, making them some of the strongest troops on the battlefield, but they are slow-moving, and take double wounds from flaming weapons or fire-based spells.
Carrion make good troops to attack your enemy's flank and rear areas, they are good for going after your opponent's artillary.
Wraiths are very deadly, can move through any terrain, ignoring the effects of the terrain on movement, they cause terror, and they have a special Chill attack. A unit of 4-5 Wraiths make a good line-breaking unit, use them to punch through your enemy's line or to roll up a flank.
Screaming Skull Catapults are good in that they have a Strength 7, and cause any unit that takes casualties from it to take an immediate panic test.
Undead Chariots with scythes are good for running down your opponent's infantry.
An Undead army must use large units of its cheaper troops backed by smaller units of such troops as Ghosts, Wights, Wraiths, and Mummies.
Tie down your enemy with your large blocks of cheap troops, and then hit him in the flanks of his units with your more powerful units. You should be able to outnumber most foes, and you must use this to your advantage. Try to deploy in such a way that you will be able to threaten your opponent's flanks, use such troops as Skeleton Warriors, and other cheap troops to freeze your opponent's line into place, so that you will be able to maneuver. Remember, most of your troops will be unable to march move, the ones that can should be deployed as a mobile reserve and/or stike/counter-strike force. This is the classic horde army, with special abilities and weakness added. You must take advantage of your Strengths, including psychological effects, to win. You must not let your opponent get into a position to flank you or to attack your general, it is always a good idea wit this army to take as many dispel scrolls as you can; if you have to, purchase extra magic users for their dispel ability, so that you can protect your general from your opponent's magic-users' attacks.
Two good formations for this army are the Horn formation and the Standard Line. If, by some chance, you're fighting an army that, due to speed or numbers, would be able to flank you, use the Echelon formation, with a unit of Mummies as your anchor, and pivot your whole line so that you are fighting down the length of the table instead of across the width of it. By doing this, you keep your opponent from flanking you, while at the same time you are maneuvering onto his flank. Always use terror and fear to your advantage, use every instance that you can cause a panic test to do so.
One good tactic: Take a unit of 24 Skeleton Warriors, formed four wide and six deep, include in the front row a Wight Champion, and two Wight Lords, plus give the unit's standard bearer a magic standard.
Skeleton Horsemen: Take a Wraith Champion, have two Wight Lords joined to the unit, and the Army Battle Standard Bearer with a magic standard, equip all of them with light armour, shield, and lance, make this unit 16-20 strong, and in four ranks, also give the unit's standard bearer a magic standard. Causes terror (the Wraith Champion), can discoporate and move through any terrain, charges with a +2S, cannot be broken, and are immune to all psychology, and even if the Wraith Champion gets killed, they still cause fear, this makes a very powerful and very maneuverable unit.
This covers the basics of fighting with an Undead Army, from this point it is a matter of gaining experience with your troops and finding the combinations that work best for you.