As far as I know the most plausible way to time travel is to open a worm hole and then sometime later open a connecting one which should allow you to travel back to the point the first one was opened. Other than that I'm not sure about going further back or into the future. Well I suppose you could go into the future through that same wormhole if someone opened a connecting one up later on.
I think there are infinite timelines based on every little change leaving the future as fluid. So say a psychic can accurately predict something, one should be able to change that future from happening.
Getting rid of time travel was the best move Heroes ever made.