Generally you have to do well in the class you want to teach. Then you can talk to the professor/dept head for that course. That's how I got my TA job. Honestly, it wasn't a major part of my application. I don't think I was ever asked about it in an interview.
The pay is modest. I made $10/hr or so, minus taxes and it was capped at 20hrs a week. But it did buy some groceries which was good. I taught a Gen Chem lab. Nothing too rough.
The pros are that you get some good teaching experience and it can be fun. Money is always good.
The cons are that Undergrads aren't always the most enthusiastic learners. Esp if you're teaching a freshman class and it's for non-majors. You have to grade assignments, you're responsible for keeping the scores and keeping track of everyone's attendance. Also, someone always either spills acid on themself or sets themself on fire.