I need help with this.?

Alright, well there's this girl at school named Krista, and I really like her. We've went out before but it never turned out well because it seemed like she was always busy. Now she seems like she's not interested in me. When I try to talk to her, my head almost goes blank because I'm so nervous and afraid if I say one wrong thing I'll completely mess up everything. Idk how to keep our conversations going that long because of that. I'm not that good with girls, I've never kissed one and I was hoping this could help. Anyone who reads this please get back to me because I need your help. Thanks.