The inside you should leave. That's seasoning and you want that in there. Sure you can clean the grate, but leave the rest alone. Short of vacuuming out the "Cold" ash out of the fire pot nothing else needs to be done.
If you really must clean fire the thing up with a dish of vinegar and a half lemon cut side down in the vinegar. This will steam clean and loosen all the good seasoning off that most smokers want.