I'm going to tell a guy I'm not interested
How does telling him you're not interested make you feel?
I feel a bit afraid, but also a bit excited
It's good that you're going to tell a guy you're not interested even if you are a bit nervous about it. Being truthful about your feelings for another person is very important, because lying about them always leads to bad results. It's better for both of you if you're honest.
I already have a boyfriend anyway.
You could bring that up so as to soften the rejection a little.