A fundamental rule that governs the universe is that causes happen before effects, not the other way around. If it was the other way around, then things could make themselves impossible. Travelling in time to the past suffers from the problem of paradox, which is much like audio feedback when sound waves from the speakers enter the microphone, go into a fatal loop and progressively collide faster and faster and louder and louder. If not corrected, the sound system will destroy itself, along with your eardrums. With time travel to the past the problem becomes radiation feedback, which would cause essentially the same sort of result.
Einstein postulated that time is like a river. It flows in only one direction but at wildly different speeds in different places. His proof shows that time travel is, indeed, possible, but only into the future, and may be achieved in one of two ways: physically travelling in an area of extreme gravity or travelling close to c, the speed of light. Stephen Hawking expanded on the concept recently by calculating that a really, really big spacecraft with a huge amount of fuel could attain speeds approaching c after about six years' of acceleration. But it would begin to travel in time after only 4 years. When the ship attained .99 c, for every day on board the spaceship an entire year will have passed back on Earth. If the course of the ship was circular and the crew started its decelleration sequence after, say, three months, by the time they returned to their starting point they would have been in space for a little over 12 years but would step foot on Earth about 150 years after they left.
You can never go back, but someday you may be able to go forward. Don't hold your breath, though.