How 2 Fix Videos
First it helps to understand how air-conditioning works.

There are 4 basic parts, there are more but this is what makes it cold. The compressor; is as it sounds, most have small pistons that compress the freon. The condenser coil; cools the freon. The expansion valve; flashes the freon. The evaporator coil is what gets cold.  The first thing that happens is the compressor compresses the freon gas. When the freon get compressed it turns to a hot gas. Then it moves to the condenser coil (in front of the engine) and gets cooled from the engine fan or air going through the coil. When the gas is cooled it turns to a warm liquid. It then go through the expansion valve. Some cars have a orifice, both make the freon flash. The difference is a expansion valve can open and shut to adjust flow a orifice will not. When the freon flashes it gets cold. The freon becomes a cold gas. This process is similar to a fuel injector spraying fuel.  Then it moves to the evaporator coil and the fan motor blows air through it and the cold comes out the vents.

How to add freon a automotive air-conditioning

There is no air compressor filter in the world that can remove all the moisture. If your system is low, and this happens more often with age. You can try adding freon to it. You are best to get a can with a gauge so you can tell what is going on. What you have to be careful about is, as we all know liquid does not compress. If you shake the can of freon you can tell there is liquid in them. If you dump to much in at one time the liquid can go to the compressor and destroy it. The way to do it is, turn on the car /truck turn on the A/C and fan on high. Then slowly add the freon.
Keep checking the vent, when it is right you should have about 30 – 35 psi on the gauge and cold coming out of the vent. This is one of those things, to much freon and it will not work to little and it will not work.

This is one way to add freon to a car or truck.

We all know they sell freon at all auto parts stores. I have used these and had good results. The first thing, is the system low or is it empty. If it is empty it probably has a leak and you need to find the leak and fix it first. You can do this by pressurising the system with nitrogen and then use a soap bubble leak finder. Never use air, air has moisture in it and will damage if not destroy your air-conditioning system. Even very small amounts can hurt and it is very hard to get out. Nitrogen is basically air with no moisture in it.

Small but there is still some risk involved. I have seen were people put to much freon in and hoses break or the compressor has a high pressure valve that will open and let all the freon out. If you add your self just go slow, add a little high idle the car / truck and add more give it some time for the freon to work through the system. Once you do this and have success you fell much better the next time you do it.
Now we are going to talk about the correct way to charge a air-conditioning. First you need to evacuate any freon that is left in the system. Then you need to use a vacuum pump to remove any moisture from the system. Then you need to weigh the freon in. If you look under the hood there will be a sticker with the amount of freon the system takes for that car or truck. They are all different it depends on how big the system is. You say you don't have a couple of thousand dollars to spend on tools and want to do your self, read on.