This can be quite complicated depending on what your trying to do. The following assumes your doing this from a Windows based computer.
If you just want to remove a normally installed program you can either find the "Uninstall" tab within the program itself. Or you can open the Control Panel and select the add or remove programs icon. A list will appear, you select the program you want to remove, you select the remove or repair tab and it is removed from your system.
Some removals require that you reboot your system after a removal.
If you want to remove a virus or spyware, malware from you system then this is a more difficult problem. The best way to remove these kinds of programs is to use a virus removal program like AVG, McAfee or Norton virus detection programs.
Any other techniques require an extensive knowledge of the Registry and Startup processes that are too complicated to discuss here.