What should I be doing with the other hand?

If you're right-handed, the classic thing to do with your left hand while masturbating is to hold or rub your scrotum (SCROH-tum), the bag of skin under your penis containing the testicles (TES-ti-culls, commonly called "balls" or "nuts"). It's also fun to let your hand go roving over your body, as you find new places to stimulate or tickle while giving yourself pleasure.