1. Check your coolant
2. Replace your thermostat
There are so many different things it could be. Take it from someone who had a bad overheating issue that took over $1400 in parts replacements (including shop labor) to track down. Problems it could be include:
Thermostat, water pump, fan clutch, radiator, hoses, coolant temp sensor, bad/dirty coolant, heater core, thermostat housing, so on and so forth.
We need MORE details, let us know what you've replaced, what you know is good, etc....it will help in a diagnosis. Is your check engine light on?