Valentine's day, ask her out or not?

I know her for two years (but not like knows her alot), but I really like her, RECENTLY. we dont usually hang out. She came to my birthday party and we went out for movie & dinner once. Some times she asked me in the hall way like how's my day and what's sup. (We dont really have any deeper conversations)I heard from people that she doesnt take serious relationship with guys. and she hasnt date a girl, but she admit that she's bi. I dont know how to ask her out and tell her that I like her. (Sorry. this my first time I ask a grl out, I'm a grl(bi). I'm really nervous and scared to get hurt. Because I have experience to liked a straight girl for 3 years and I got so hurt when we stopped to be friends. So right now I'm really scared to ask this girl out.)