There are two directions to go with time travel: science fiction or fantasy. Fantasy is a lot easier, but it's less challenging to the reader and writer, so I'll assume you're going with science.
There are several schools of thought on time travel:
1) It's impossible. The consensus between most physicists is that changing the rate at which humans go through time is simply impossible. The flow of time may not be a constant, but the manipulation of time, in theory, would require more energy than one can even imagine, let alone generate. Think of it this way: the sun has a mass of about 20 x 10^10 kg (roughly). The gravity well it creates stretches far, far beyond the solar system. One would need an atomic clock to measure the time change created by the sun, a normal clock couldn't do it.
2) Go really, really fast. Because time slows as an object comes closer to the speed of light, some scientists (but mostly sci-fi writers) theorize that moving faster than light could propel one backwards in time. This main issues with this are that breaking the speed of light is a touchy subject with physicists. I actually saw a professor of astronomy knifed once over it by a roving band of resident physicists, it's that serious. The only theoretical way for matter to break the speed of light is by manipulating gravity by an obscene amount, which would (referencing #1) require gobs of energy. The most popular instance in fiction of this warping of space via gravity is the warp drive of Star Trek. The energy used in the fictional process is gathered from colliding matter with anti-matter and channeling that energy through a fictional crystal which somehow translates the incredible energy that would theoretically be released from the matter/anti-matter explosion into some form of energy that can be used. That translated energy is sent to the "warp coils" (another totally fictional tech) which somehow generate an incredibly powerful and easily controlled warp bubble. That bubble bends space, making space move faster than light. In order to make this work in a science fiction, you would need to use creative license with the tech that could generate that effect.
3) Quantum theory. Quantum theory is crazy. Even with the recent introduction of M theory, it's still a big mess that we barely understand. One of the most fascinating aspects of quantum theory is the instantaneous transfer of information. This information clearly moves much faster than light, and as such could theoretically, when applied to #2, move outside of time. When we develop a better understanding of this process, we may be able to harness it, but for the time being it's probably far to complex to be used without some artistic license. Also, considering the rate at which this particular area of science is growing, it's possible that your writing's science may become obsolete quickly.
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