For all Hordes armies the best place to start is actually to get the starter box, Legion is no exception. Then the Primal book of course, but that's something you can borrow at first (same goes for the expansion Evolution). From that point on I'd suggest the following steps:
-try a few games with the starter to get a feel of what you particularly like in the army you picked;
-the book has the profiles for all models in all armies, so you can actually play models you don't have in friendly games to see whether you like them or not;
-check out Battle College
for general info on every single model in the game. Nothing written there should be considered as gospel, but it gives a first opinion which is always worth taking.
But really the first step is the most important. You'll already have fun games with what's in the starter.