I got asked out by a girl for the first time.?

Well, here's the story. I'm a guy, 13 years old, I was just walking around normally in my school towards the lunch hall, then I was stopped by a girl in my cl. She said these words "Will you go out with her? The girl over there?" She pointed to a kind of hot girl standing near the wall at the other side of the corridor. I didn't really know this girl, but I did like her, I was kind of embared and didn't know what to do - I'm quite shy really and this is one of the first times I've been asked out. But because it was so fast and an on the spot decision, I said no. She looked sad but I did want to go out with her but didn't really have the guts to say it. As I said I don't really know her but I would go out with her if I could. I get bullied at school, though. I don't know if when I'd be hanging out with her if I'd get ripped on or showed up in front of her. I took these into consideration when I said no. I'm not really big and I couldn't stick up for her very well like a guy should do, I'm 5 foot 4 ish so that's kind of another thing that held me back. I just want to know what to do - give it another chance? If she asks again, should I accept the offer?