How do I tell my best friend that I love her?

Alright, I'm on my friend Jess's account, but I'm not a girl. My name is Adam. I want to go out with my best friend. I've been friends with her for four years and liked her for three. She's been away all summer at camp, and I hardly ever see her at school so I decided to throw her a surprise party. I got her a present, it's a heart necklace that has "My only sunshine" engraved in the back. First off, do you think she'll like that? Girls, if a guy gave you that as a present would you like it? After I give her the present, I plan to tell her my feelings. I just don't know what to say. She has a boyfriend, they've been going out for about a year and a half. He has cheated on her twice, I know but she doesn't. He shouldn't be going out with her, he doesn't deserve it. He's not invited to the party, he actually has football practice right in the middle of the party. (Not like I planned that or anything... pfft.) So yeahh, tips on the gift and how to tell her?