Great questions. When I first started brewing, my local forum recommended I buy "Designing Great Beers" by Ray Daniels. It talks about all the aspects of grain, hops, water etc. and how to combine them into recipes. That's the first half of the book. The second half has chapters on various styles of beer and how to make recipes for those.
While I was reading and trying to understand that info, I did as someone mentioned already: I used others' recipes from the various forums. Over time, you get the hang of which grains to combine based on what type of beer profile you are after. It just takes time and practice. The journey is the best part because either way, you make beer!
Good luck,
Baz