I am basically being forced to go to Prom with a girl i can't stand! I will be ridiculed for it!!!!?

Okay i'm going to make this as short as possible. I know its early but my school does its prom payments early in installments so that everyone can afford it. This means everyone needs to make their plans really early and stuff. Now i have never had much luck with girls and i have a friend who is a girl who i flirt with but that it really. Now i have this sorta friend. She's overweight, not attractive to me, and really bossy and obsessive. I have never liked her even as a person. The only reason i consider her a friend is because she's a part of my group of friends....anyways she's been hinting on going to prom with me for a while and i could tell so i've been advoiding. See she had a boyfriend actually a finace who was in the army so she couldn't go to prom with him but they just broke up. So now she was looking for a prom date. I asked the girl i liked but she's going with her 'boyfriend' so now i knew i wouldn't have a date. Today i was but in a awkward position by a friend who basically brought up the prom situation and made the girl i don't like...officially ask me to the prom....I couldn't avoid it!!! i had to say yes or no...and i said yes because i couldn't say no. I mean i understand that i don't have a date but my friends are going to mock me the entire year for going to prom with her. And i can't even stand her. Help me! the only reason why i said yes was because i didn't have a date and i could be my one chance! but i don't want to go with her. and when my family (cousins....aunts...uncles) see my date on the night of the prom they are going to laugh...not the adults but my cousins. See...my brother went to prom a few years ago and his date was really pretty and she doesn't even compare to my date now....what do i do!!! And i'm also scared that she wants to be more than friends and i don't want to even be friends with her!!! HELP ME!!!