why does my heater only blow out cold air

You could have an engine that does not get hot enough, or a problem inside the car that blends the termperature of the air coming from your dash. First check to see if the engine is hot - check the inlet and outlet hoses of the heater core - are they untouchable hot? Both? Just one? Neither? (be very very very careful not to get burned...leave this all to a qualified technician to err on the side of safety...) Both hot is what you are looking for. One hot means a possible heater core restriction. Both cool could mean a thermostat sticking open. If the engine is hot enough, then check inside. You could have a temperature blend door motor problem, or a crack in the plenum case where that door operates. This repair would require removing the dash. In most cases, all of the above probably needs expert attention - but you can at least learn and check some things along the way. Good luck!