How do i tell my best friend that i LOVE her?

Ok. so ive been friends with this girl for a while now, and i'm falling for her. she know's that i LIKE her... but i need to tell her that i love her. i mean, tonight we were hanging out... and she randomly took me to meet her dad and we had dinner over there and it was amazing. my real issue is that she has a boyfriend. i didnt like this guy before they were going out. i didnt like him before i knew that they even knew eachother. so after this great day we had tonight he told my friend that he cant go to prom. and i was talking to her and told her that id love to take her. so were going this saturday =] but like i said before, i dont know how to tell her that i LOVE her... shes the reason i look forward to getting up in the morning and the reason i take 2 busses to school everyday... how am i supposed to tell her this stuff? i dont want our relationship to be all messed up... or wierd... i just.. i need to know if theres any chance she feels the same way...