First you should find out if the hose for radiator fluid going to the heater core is actually hot or not. If both hoses are not hot, then you have a problem. It might be a blockage to the heater core. You'll have to check and see, but be careful you don't scauld the hell out of your hands.
If you notice that the hose going in is not hot, it may be possible to flush out a blockage using your garden hose (but again, don't burn yourself). But be sure that this is actually the problem before you do anything. If you aren't sure, then visit a mechanic.
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