If your were a teenager how would you let a girl know you like her?(even if you have a girlfriend)

so you have a girlfriend but you like another girl? Dude you gotta pick one before you tell anyone anything.

If you already have a girlfriend, either you don't tell her, or you break up with your girlfriend first.

if you like the girl a lot then you break up with your old girlfriend and when it is valentine you and give her a flower and write some that tells her that you like her and if that same old school then get up the to her and tell her that you like her