I need help approaching my crush?

[Hey Everyone]

I'm 15 and a freshman in high school. I recently started liking this girl in my math class even though I've never talked to her. I want to ask her for her number or something like that so I can get to know her but I'm really scared to go up to her. I think I'm a pretty attractive guy but I'm just not confident, I feel as if I'm gonna get rejected and even though I know that's not a big deal it just scares me. I know people are gonna say things such as "just do it" but please try to give me actual advice. I love playing soccer and I know she plays soccer too because I saw her practicing with the girls team while I was at practice so maybe that could be something to talk about. What I'm asking for is just tips or ideas as to how I can approach her and ask for her number without just walking right up to her and doing it. I don't really sit by her in math and that's the only time of the day when I see her. Any advice? Thanks in advance guys.