These problems could be caused by a few things. Let's go from simple to difficult. (1) Air in the coolant line. You need to "burp" the line by removing the pressure cap from the expansion bottle and running the engine for a few minutes. Put the top back on and run the engine warm for a few minutes. Let cool and repeat the process a couple of times. (2) Stuck actuator switch. Look under dashboard behind glove box. There is a small white box on the heater assembly, with an actuator arm coming out of the top of it. Turn your heater on and off, and see if the arm moves. If not, tapping might free it, or you might need to replace it. (3)
Dirty, clogged, or bad heater core. Watch this YouTube video
http://www.youtube.com/watch?v=LsZoFoiJsdw