Security Clearance & Internet Usage?

I have been looking at positions in the JAG area of the military and see that it require a security clearance. What level of clearance do I need? Confidential? Secret? Top Secret? From what I have gathered I don't have many problems but I am a little wary of them looking at my "internet usage". Can they read all of my old email and instant message history even if it has been deleted? What exactly are they looking for with this? I don't have anything illegal on my computer so maybe I am just being paranoid. Maybe I don't want to be embarred if they said something like "someone likes to watch ..." lol. Any help or comments would really be appreciated.