The best way to start
TK's is to get their battle box along with the army book and rule book to start.
As far as units that you'll need, you'll need to get some Core (skelletons, horsemen, and chariots - all which can be found in the big battalion box).
Next, you're going to need at least 1 Liche Priest and one Tomb King or Prince. For these, you have choices - you can get the empire wizards box and put tomb king heads on them from the battalion box (you can find a couple of really cool conversions on the interwebs for this). For your King, you can get the big Sphinx Box and make it the elite unit that has the Tomb Guard unit. This will give you a character and special choice all in one.
Get a couple of games in at this point so you can then decide where you want to take the army - more magic (Casket), stompy (Bone Giant), or shooty (catapult). After you get a good core and special units you need to see where your play style is and go from there. For the most part, you'll have a good shell for an army and will be able to field a solid army.
Automatically Appended Next Post: Here is a good blog and it shows how he made some unit fillers so you don't have to have so many models. All it takes is some Hirst molds. I like his conversions on how he made his liche priests as well.
http://www.dakkadakka.com/dakkaforum/posts/list/365225.page