Minimal hot air

Low coolant level will prevent the heater core from working. The heater core is of course the thing that provides cabin heat. The best way to find a coolant leak is to pop the hood and just get in there and look. Check the hose connections, run your hands along them feeling for wetness. Check the radiator itself, if it is leaking you may see coolant on the support. Check the ground after you've been parked for a while for coolant. Check the front of the engine (the side with the serpentine belt) for leaks near the water pump. One leak source that is hard to find is with the heater core itself, as that is inside the dash. If that is leaking, coolant may flow from the overflow tube to the ground. However the heater core may only be leaking when the car is running, so it won't always be obvious. The last and worst case scenario for losing coolant is a blown head gasket, but usually you have many more symptoms for those than you are describing here. The cost of repairing a leaky cooling system at a shop is fairly minimal so if not something you can do yourself, take it in somewhere. Don't let it get to the point of overheating the engine.

I guess my next question would be, if I just topped off the coolant and the temp gauge on the dash still isn't moving and it's still giving off warmish heat, what would I want to look at? Obviously fixing the leak is something I need to do. I guess my understand at this point is if there's coolant/antifreeze, it should provide heat and the temp gauge should still move, so shouldn't it work properly if the coolant is filled up?

Even if you have coolant, it doesn't mean there is coolant flow or at least adequate coolant flow, if the water pump if beginning to fail. Next time you have car running and warmed up, try squeezing the heater core hoses (these are about 1" in diameter and run to the firewall. They should be hot or warm to the touch and feel pressurized. If they're not, then you have no coolant flow to the heater core. If you think the cooling system is working (even with the leak) and still no heat, you can try a flow test on the heater core. If cooling system AND heater core are working, you may have a problem with the temperature blend door, which directs are across heater core or AC coil, depending on what temp setting you select.