I am in love what do i do.?

i really DO love this girl but she has a boyfriend. and i havent dated her. also i have liked her for 4 years and i want to date her so what do i do? and i dont stock her i swear.but really what do i do? do i ask her out or what? and i dont know if she likes me ooo and one more thing we are both girls. but i really do LOVE HER!!