He thinks I'll say yes to him but I don't like him anymore?

I know, it was my fault for telling him that I like him, but that was like 2 or 3 weeks ago, and yes, it's bad that I get tired of guys easily. But now this guy likes me and I don't like him anymore. We went to the movies together and it went horrible for me. He's gonna ask me out soon and I dont know what to do because he obviously thinks I'm gonna say yes to him, but I wanna say no. How should I tell him when he aks me out?