How to tell a girl that you want only to stay as friends?

Might as well tell it to her straight-out, yeah. I'm pretty laid-back irl, so when I say something, even if sarcastically, most people won't seem to get it unless they're close friends. So yeah, I think I'll tell her before she thinks I like her or something. She's nice, I mean, seriously. Super cute. I'm just not interested. ._. Thanks for the advice, guys!