Eden200

I'm a bit of a noob :/ How do I do this?

When I originally bought ARMA 2, I believed there maps w/ enemies and there were no real objectives. (In other words, not a campaign/mission, but a sandbox mode). Maybe something like a sandbox mode, but I can request missions from a commander, I guess. Is there a sandbox mode/something close to it with randomly generates enemies and the such? Or is there something like it?