There is this guy who likes me a lot but...?

So clearly he thinks he would have a chance if you sat down and thought about it, so tell him that you WILL think about it, but for you to have a clear mind and make the right decision he has to give you space while you're making your decision. So say, "starting now I'm going to decide whether or not I want to pursue a relationship with you. Don't contact me until I contact you, and when I contact you I'll tell you my decision." From there, you can sort out your feelings and rethink whether or not you want to be with this guy. In a few days if your feelings haven't changed you can just say "look, I've thought it over for a few days and I really just see you as a friend. I don't want to wreck our friendship because it's really important to me. So lets stay close in a friendly way." or something along those lines. Make it clear at that point that you will not rethink your decision, because you've already had 3 straight days to think about it.