GIRLS: how do I know if she likes me? How do I ask her out?

I have deplorable self confidence after being bullied in middle school and rejected by every girl I have liked. I try not to give up, not to be so down on myself, but it's hard to feel like I'm attractive to girl and it seems like i'll never have a girlfriend. Currently, I'm a senior in high school, about to graduate, and there's a girl I talk to that I like a lot. She told me, in a very unclear way, that she wasn't interested in dating me, but she really makes it hard for me to believe her because of when she flirts with me and all. I have never had a girlfriend, but I feel like now I may have a chance with her. Either way, I want to hang out with her sometime again to tell her how I feel and to see where she's at mentally, but I am terrified of asking her out again after last time... How do I 1) get my self esteem and confidence up 2) muster up the courage to ask her out and 3) become better with girls? Thank You.