All right, I know that time travel is confusing for a lot of people. Hell, I'm a Time Lord with a doctorate in chronodisplacement theory, and it still gives me headaches. I can, however, help with two simple guidelines.

1: History can be changed.
2: Your own personal timeline cannot be changed.

Those are more or less the two main rules of time travel. Now, people ask me all the time, why not just stop Hitler, or why not just stop whoever the big bad guy in their universe is. It's not that simple. Kill Hitler, someone else will take his place, and maybe he wins the war for the Axis. Save your dad from dying, and the murderer gets your mum instead. History is funny like that.

In general, I try not to purposely muck with history too much. Unless it would be funny.

Now, your own, subjective, personal experience should NEVER be changed. That means if you try to meet yourself, bad things will happen, unless you already met yourself, in which case if you DON'T try to meet yourself, bad things will happen (time monkeys). However, this doesn't mean you can't change history you've seen - you can.

Oh, and if you do end up time traveling, try not to give away the secret. There's a reason we don't just let everyone run around time all willy-nilly.