/jp/ - Otaku Culture

I'm studying from the Genki II textbook and I don't understand certain things with particles.

1. How do I know when to use はor が? Often times I fuck up by writing ”私は” instead of ”私が”

2. Why is の used in the sentence ”文句を言うのをやめなさい。”? Is it a requirement to use の right after a verb (in this situation) when followed by another verb?

3. ”帰ってきてほしい” means "I want you to return," but why isn't it just "帰ってほしい”? The first one is from the text.

4. What does ”なった” signify from ”行けなくなった”?
From the text:
”それに「急に出張が入って大阪に行けなくなった」という研一の伝言が留守番電話に残されていた。”