Say these words: "Hi. Um, yeah, you know how you said that you liked me, well um I'm not that kind of girl and I do not want to date you. Sorry, but I am not willing to do that." Then smile and walk away.
be nice about it.. tell her that she awesome, you think she funny and nice and awesome to be with but you just dont feel like that towards her and you are straight so you never will. make sure she knows that its not a "at the moment" feeling but you will never but stilll like her a lot as a friend.
First of all ask er if she was being serious and then if she says yes then tell her hey um... I;m sorry but I like _______( except say crush's name in the blank) and say I'm sorry that I don't feel the same.But if it was a joke then just laugh it off.
just be very straight forward with her say something like this im sorry i dont feel that way about you i only like boys and im not ever going to be into girls unless im like super wasted lol jk dont say that last part
you shouldn't treat her differently to a boy who you didn't want to go out with. If you wanted to say 'no' to a boy nicely, you'd say something like sorry but no, say the same to her, she'll understand. I know because i have a by friend and she's asked my friend out occasional times, if you say no politely, you'll be okay
say really politely "im sorry, but i like u as a friend and it would b really awkward for me to b in a relationship with u... but i still want to b friends. can we still b just friends?" and then things should be all good. even if she gets her feelings hurt, she will get over it in time.