What should I do?

Ok, so I'm a junior in highschool and I really like this girl in my English class. I've been talking to her a lot lately but I would really like to take the next step and hang out with her. I want to ask her but I feel like if I ask her too soon and she says no that would ruin my chances for good but if I wait too long maybe she won't be available... What should I do?

You left out the most critical information: what she might think about it. You must have some idea whether she's interested or not. If she seems interested in you, ask her now. If she doesn't, wait until she shows some interest. If you have a chance to work with her on a class project or can ask her to help you with English homework, that would be a good move..