Ermm, so, a boy asked me out but I'm not interested in him,how can I say no but not hurt him too much and still be friends??

Here is the most simple thing that you can say. "Let me think about it" That usually works and then he probably won't ask again. If he does then you will have to say you just aren't ready for a relationship at the time.

Be honest, it's going to hurt no matter what. Just say "I'm sorry I don't feel the same way about you, I see you as a friend and value that friendship."

Say something like, "I'm so honored that you asked, you mean so much to me as a friend. I'd rather not change what we have, if that's ok with you?"

Like Rome said, just be honest with him.

Being honest is the way to go, but he may be hurt. That the chance you take asking someone out.

Be honest,and if he's hurt as he probably will be,that's what you risk setting yourself up for when you ask someone out.
We all get rejected at one point or another.

Be completely honest with him tell him that you like him as a friend and you don't want that friendship to be damaged. If he has feelings for you, it might be better not to tell him about your relationships in the future. I know this personally because I was friends with a girl I liked, she told me how much she loved this boy and another boy and it hurt me inside. If you do that to him it'll tear him apart inside.

I enjoy hanging out with you, but I don't want to go down that road with you.

I really appreciate that you asked and I enjoy spending time with you but I'd prefer it if we were just friends.
It's going to hurt no matter what you say but it's a lot better than leading him on and humiliating him.