How do i tell her? i can't keep it a secret forever?

i have this best friend, i love her! she's the only friend i have that i could NEVER get tired of. but there is this guy, we were realy close friends, talked on the phone ALL the time but i never really ever thought of him as more than a friend. and my best friend happens to like this boy. she'd always be like "ariel! tell me if he asked you out YOU would say no?!" and i was like "Yeah! i would say no cuz id NEVER think of him as more than a friend. thats gross" and lately the guy has been talking about french kissing me and stuff and i would just think he's joking around. but i didn't think he'd do this... but he asked me out :O i was so surprised i didnt believe it, then finally i found out it was true. he liked me. so i told him id call him back. i was planning to say no but i thought about it and gave him a chance. and it all led to me falling for him. so now we've been dating for a week and ive still have been hiding it from her, I CANT LIVE THIS LIE! i have to tell her, but i know she'll call me a hippocrit and a ***** cause she likes him. i know i'm stupid! but i cannot let go now. i need to tell her in a way that she wont get too mad, maybe an excuse? or something, just something that will allow me to be seen with him without hiding it from her. how do i tell her? help me, please :[ i cant lose my best friend now...