Here are 2 very informative links that will answer many of your A/C questions:
http://www.automedia.com/Maintaining_and_Recharging_Your_AC/ccr20050901ac/1
http://www.automedia.com/Air_Conditioning_Troubleshooting_I/ccr20020901ac/1
An air conditioning system is not too complex, but in order to do any real repairs to it, some expensive pieces of equipment are required. A/C systems are pressurized, so how does one empty the contents of a can into a pressurized system? The quick answer is that you can't. Here is the problem: A properly functioning system will never lose refridgerant, but when the system no longer functions the way it should, it is either because the refridgerant has leaked out, in which case the leak must be repaired for the system to ever work properly, or another component has failed or become plugged. If a component has failed or become plugged, putting more refridgerant into it will not help, and would be pretty impossible to do anyway since if a component failed, the system would still be pressurized. If the reason the system isn't working is because of a leak, than the system would no longer be pressurized, so you may get some refridgerant into it, but it will just leak back out. And if you need to put more than one can in it, do you think the second can you attempt to put into it will be able to overcome the pressure in the system present from the first can? So how does a shop do this? They hook a machine up to your car's system that puts the system under a vacuum. When a vacuum is created in the system, it will suck refridgerant into it instead of trying to force it in. Another benefit of this is that if there is a leak, the system will not hold vacuum, and then the original problem, which if you are out of refridgerant is a leak, can be found and repaired. If the system holds a vacuum, then you can be assured it will hold refridgerant. Another reason the proper refridgerant is so important is that the most expensive component of an A/C system is the compresor. This compressor needs lubrication to keep it from freezing up, resulting in a costly repair. It gets this lubrication from oil that is put into the system when it is charged and is passed through the system with the refridgerant. If your system has a leak and is losing refridgerant, than it is also most likely losing oil as well. When you install a can of refridgerant yourself, you are not replacing this oil, and are actually helping more of it to be leaked from the system. This is why it is best to let a shop with the proper equipment and ASE certified techs who know how to use it repair your A/C system. I have been doing my own repairs for 20 years, and although I may replace my own components, I will still take my vehicle to an ASE certified shop to suck down, leak test, and charge the A/C system.
One other thing worth noting is that some of the replacement refridgerants being sold are just plain dangerous. Some of them include ether, propane, and butane, which are all flamable and can cause health problems. In the case of ether, it can render someone unconscious. If your system needs refridgerant, than it has a leak somewhere. Do you really want to let flammable gasses leak into a hot engine compartment, or if the leak is in the system's evaporator, which is often times located inside the car, into the inside where you and your passengers may be sitting?
There are many repair and maintenance procedures that owners can, and in some cases should, perform themselves, but working with refridgerant is not one of them. Here it is best to err on the side of caution and take your car to a qualified repair shop.