How do I politely reject to being someone's valentine?

Yeah, I'm terrible at this. I just wanted to know is being a valentine like a prelude to relationship? The guy who asked me has liked me a really long time, and he's good and all, but I'm just happy being 'solo' at the moment. I don't want to hurt his feelings, so how I can politely say no? Or should I just not worry about it? I'm this desperate, it's obvious I know.