I would use a time machine to go back and tell my younger self the specific stupid things that I did as a kid (and adult!), and make sure that I didn't do them!
Unfortunately, with our present understanding of space-time, there is no such thing as backwards time travel. There really isn't anything such as forward time travel, either. The best you could do would be to build a pod and accelerate to extreme speeds (the faster the better; past the speed of light and beyond). Your relative time will continue at its pace, while the relative time of the rest of the universe will pass at a faster rate relative to your time frame.
So you could "travel into the future" by skipping over large chunks of time. But you could never get back, it's a one-way trip!
Unfortunately, we do not currently have the technology to develop a machine that travels FTL.
I guess right now the best you can do is sleep and let time pass while you're unconscious. ;-)