Should i keep my Twitter account?

I was looking forward to it because i hear it's so great,I'm not a stalker or anything like that but i was hoping to tweet famous people and it turns out they need to follow u in order to message them or something like that. So I'm not searching for anyone in particular to tweet but my questions are: y can't u tweet people who don't follow you, is there a way to make certain people on twitter follow you, and finally is it all really worth it if u can't tweet the people you want to. I'm new to twitter so I'm just asking those general questions.