Total noob question on army building.

I have bought the 8th edition core rule book, as well as the 2nd Imperial Codex and a few boxing of various IG stuff. A command squad, an infantry box, a heavy weapons squad box and a box of Scions.

Looking through the core book and the codex I don't really see any instruction on how to go about building my army. Are there certain units that are mandatory? Is there a minimum of certain things? Differences based on the points value I'm shooting for?

Also I really have no idea about how I should be outfitting my guys. What gear works? Or is everything fairly viable?

Am I overlooking this stuff and it's in the books?