1) I'd go back in time several decades and kill the inventor of the time machine while he's asleep in his crib. The probability that somebody will do something that causes a severe time paradox is too great to allow such a dangerous trinket to exist.

2) I'd probably just go 50 years into the future and check up on myself so I know what to do and what not to do later on. Even if I'm dead by then, at least I know how I died and how I can prevent it from happening~