I just replaced my throttle body with a stock new one. I started up my car and the idol was very low. It was sitting at about 500 RPM. How should I reset the timing?

 

Idle RPM and tiiming are two entirely different things. Moreover, there's a good chance that one of the problems you are experiencing may be an incorrect TPS setting.

Perform a search for "TPS and Minimum Air Position" setting procedures. If you need to check timing, you can find lots of information on that by searching as well.