As others have said you'll need the Index Xenos 1 book, which contains the Eldar datasheets.
Right now you have two HQ units (the two Farseers), two Troops (Guardians and Dire Avengers), 2-3 Fast Attack (Vyper and Windriders - Windriders can be fielded as one unit of 6 or two of 3) and 4 Heavy Support (Fire Prism, War Walker, and each Wraithlord is a separate HS choice).
That means that as the army stands the only legal formation that allows you to use all your models (in a single detachment) is a Spearhead detachment (the only one that allows more than 3 Heavy Support with fewer than 3 Troops), and only then if all the Windriders are in a single unit.
Your options going forward then are to either:
- keep this force as it is and bulk it out (War Walkers and Vypers can both be fielded in units of up to 3, and Walkers in particular are of limited effectiveness on their own). You also have space to add additional heavy support, Elites and Wave Serpents if you want to hit a particular points threshold (or just want specific units)
- as Killersquid suggests getting a third Troops unit to field a Battalion detachment - which will allow you to expand your army quite a bit more without a second detachment. A Battalion can only use three Heavy Support units, however.
- Field this army in two detachments, as you can potentially field minimum-sized Spearhead (1 HQ + 3 Heavy Support), Outrider (1 HQ + 3 Fast Attack) or Patrol (1 HQ + 2 Troops) detachments. Outrider + Spearhead gives you two extra command points (instead of 3 with a Battalion) and a lot more extra spaces to play with in every section of the army list (up to 2 extra HQ slots, up to 4 more Troops, up to 4 Elites, up to 5 more Fast Attack, up to 4 more Heavy Support, and up to 4 Fliers). The only thing a Battalion offers more of is Elites (up to 6).
As far as immediate model purchases go, I think you'll probably want to complete the War Walker squadron - and likely give all models brightlances or magnetise them - and otherwise you're in a good position to get started. In the longer-term, as built this is a more mobile force that will likely benefit from adding an extra Wave Serpent for your other Troops unit.