Usually the culprit is low coolant. Add that 1st.

The next step is to replace the thermostat.

Could be a clogged up heater core too. Ummm, what I do is let the motor warm up to operating temp. Then pop the hood and find the heater hoses. The inlet and outlet hoses should be equally hot (just grab em with your hand, it won't burn you). If inlet is hotter than outlet, then you should replace your heater core.