First I'd fill it with water, make sure no leaks. that'll also do some of the work for you.
Then, I generally use pure white vinegar. Wipe the whole thing down inside and out, then do wipe down's with water. You could also do a bleach solution (I think Ten Percent is somethign I've seen suggested) before the vinegar. I'm wary of letting the bleach into my system but others swear by it.
Honestly if it's been dry for any length of time, it should be okay with just vinegar, or so I'd think. Just make sure to clean it with water till you can't smell the vinegar any longer, than let it dry, preferably in the sunlight. Vinegar and water are also good for cleaning the glass once you're using it with fish. Not toxic, like most glass cleaners.