Hey, guys! It's Laura! I know I'm probably bombarding you with tips right now, but I'm kind of on a roll, and this tip is especially important. @lola-gal asked for some advice on this subject a few days ago, so here's my two cents!
HOW TO TURN HIM DOWN:
There really isn't a foolproof way to do this. The only advice I can give you is that you need to talk to him. Gather your courage, get him alone, and politely tell him something along the lines of this:
"Hey, I'm really flattered that you think so highly of me, and I think you're a great guy. But I'm just not as into you as you are into me. I would still like to be your friend, just not your girlfriend. I don't want things to be awkward between us. I hope you understand."
After that, it probably will be awkward for a little while. Then it's up to the guy as to how he wants to react. If he's a good friend, he'll be understanding and respect your feelings. On the other hand, he might be totally crushed. But the thing is, there's nothing you can do about that. You did your part and was honest with him. If he wants to be offended by that, that's his problem, and chances are he wouldn't have made a very good friend OR boyfriend.
After that, just act like you normally would. Be friendly, not flirtatious, and you won't have to worry about leading him on.
Hope this helps! :)