I have a mouse (?) running around in my apartment. Suggestions?

I’m not sure if it’s a mouse or not, but it moves really fast and I can’t think of anything else like that (it’s also got a long tail). It could definitely be a rat, though.

I’ve never had to deal with rodents before—any ideas? (I live in the desert: my idea of problems are scorpions!)

I don’t want to kill it, and if it’s not going to harm me or my stuff, I’ll probably just leave it alone, but like I said—no experience.

Thanks!