do it in steps :
1. check for leaks (this means looking under neath as well)
2. if no leaks then flush the system till clean
3. if there is leaks fix em and still flush the system

if those steps do not remedy the problem then either three things went bad radiatior, thermostat, or water pump
4. i would suggest start with the thermostat if you replace that and still over heats go to next part radiator and then if that does not work go to the water pump

if you do have to replace any of these parts i would suggest replacing hoses at the same time (might as well