The first is to pick what you would like to start collecting. Obviously your EfGT set can get you going. You can decide you want to play Dwarves or Goblins and start adding to those forces. Or you can decide you'd like to start a different force all together, perhaps something based on the LotR stories instead. A really important thing is to pick an army you are interested and excited about. You will find that for the most part any army in the LotR/Hobbit range can be played competitively. There are some that would be harder in a tournament-level environment, and some are easier or harder for early players, but they're pretty much all good choices. If you pick one you really like you will have more fun and be more excited to buy/paint/play them.
If you're not sure yet, maybe play a few more games and post what you do and don't like about the game and we may be able to suggest forces that lean to what you like the most. Do you have any other local players? Often they can let you borrow some armies for a game.
Once you have an idea of what you would like to play we can help suggest what book(s) you may need, and what are some good early purchase options to get you going without going broke.
