Take off the strings, and then get some "LOVE POTION NO 9" cleaner and rub it all over. I use it and I'm always amazed at how much grime it gets off of my instruments.
I have two types (both with different numbers) one is a deep cleanser for getting off all the dirty oils and grime from the fretboard and body itself, and it's a cream you rub in on the finish. The other is a spray that puts a thin layer of protection over the newly cleaned finish, and keeps it clean and shiny for weeks.
Great products, but a bit pricey if you don't have much.
Other than that, get some soap and water, damp a cloth, and have at it, guitars aren't all that complicated and don't take much to clean. Dawn dish soap works best, if it's good enough to get oil off of penguins, then you can get some sweat off the body.
