How do I tell him?

So when you say he likes you, do you mean as a friend? Or as a girlfriend?

If he likes you as a girlfriend, it's easy! Just ask him out. When you are communicating next, say "I'm tired of being on the computer; want to come to the movies/ game/ whatever?" If you feel like this does not make it clear enough that you are asking him on a date, just add that you think he is fun. Then when you're at the movie/ game/ whatever, if he likes you, he will (hopefully) make that clear by flirting/ asking you out again sometime.

You're nervous about this and that's normal. Just remember that Josh likes you, so he probably won't be judging you. The secret to sounding like you are not nervous when you're talking to him is to sound relaxed and confident. Confident means speaking slowly (if you talk fast he'll know you are nervous) and briefly (don't babble/ramble because that's a sign of nervousness too). If you sound confident, people will treat you like what you are saying makes sense. If you sound nervous, things will feel awkward. But like I said, even if you get really shy when you are talking to Josh, he probably won't care if you do sound awkward because he likes you!

Good luck!