Hi Riot_Riot,
Firstly you have to decide whether yo just want to remain friends with her, or if you do ask her out, whether you are both mature enough to remain friends if it doesnt work out. However, you are only young and I would suggest that you dont ask her out as such, but simply give her a valentines card and maybe a single rose or bunch of flowers. In the card, write that you really like her and that you would like to go out with her. I am 22 now but when I was that age, if a guy would have done that for me and ould have thought it was really thoughtful and romantic.
Let us know what you decide and how it goes
x