How can I get this girl to give me a chance?

I met this girl, her name is sammy. We met through my friends. It turns out she is their cousin. We all went to Six Flags and had an awesome day. At the end of the day she went home and the next day I asked my friend for her number. He texted her to see if she wanted my number, not what I asked but I didn't care. She replied to him and stated "I like him, not trying to be mean, but not like that." She is pretty cute, fun, and funny. I didn't know why she turned down taking my phone number. I'm fat, but I'm a pretty sweet guy, I get along with everybody, and all the girls I talk to like me, but pretty much all of them don't like me like that. I though this girl was different but dang!, won't even take my phone number. I havn't given up on this girl, and any tips on how to get her to like me enough to take my number would be great. By the way im 16 she is 15, this isn't an adult case, so think of it like that. I can't take her to my apartment (I live in a house, this is just an example) and seduce her, like someone maybe in their twenties would do.