How do I turn a guy down nicely?

So this guy I've known for ten years just asked me out...we've been friends for a long time but I really just don't like him like that at ALL. I really don't want to hurt him and he's in my group of friends so I'm trying to refrain from awkwardness as well (but I know that really isn't possible...) I've turned down guys before but it was when I was younger so it wasn't as serious...ugh this is really stressing me out so any help would be appreciated. Ps, this is what he said: Hey I was wondering if you would go out with me.

Please help!