Sometimes my heat works but most of the time it just blows cold air.

Check water levels make sure there at the correct amount its one of theses 3 things thermostat temp sensor or heater core the engine has to be warmed up to produce heat inside of car good luck

Locate the 2 smaal heater hoses under the hood and let engine warm up. Feel both hoses while running to feel if they are both about the same temp.If not removethe hoses and clear restriction.I have found that in very cold weather that 50/50 coolant mixture very important.you can test with a coolant tester to see if antifreeze concentration is too much

Get your radiator and heater core flushed out, if this doesn't fix the issue then your contol damper is probably not functioning like it should. The heater core and radaitor should be able to get flushed at the same time and this is a good thing to do on occasion anyway. Have you check your engine thermostat? this could be the culprit if it is not closing and allowing the engine to heat up.