There could be several reasons why your heater is not getting hot.
1. Is the engine running (only joking)
2. Do you have a temperature gauge on the instrument panel and if "YES" does it read between 1/4 and 1/2 when up to normal running temp? If it reads normal but the heater is not getting hot then you more than likely have a blocked heater matrix. If it reads very low, then it may be a failed thermostat.
3. Check the temp of the hoses going to the heater by hand. If one of the hoses is too hot to touch and the other is stone cold, blocked heater matrix. If both hoses are the same temp, failed thermostat.
4. Does the interior heater fan work on all speeds.
5. Does the radiator fan cut in when the engine is hot.
Give me a few more details and I will try and help.