Just about everything: The compressor, the condensor, the evaporator, the expansion valve or capillary tubes, all of the piping, the reversing valve if it is a heat pump, the valves, the service ports, the filter/drier if there is one. Refrigeration service techs have a special 'sniffer' used to find the source of a leak.
No
If your air conditioner blows air but it's not cold air then you probably need freon in your air conditioner
$100
29 dollars for refill
Yes you can if you have the tool, knowledge, and of course the freon. Youtube has instructional videos to help you.
The answer should seem obvious... you have a leak. It could be anywhere in the system, which is unfortunate for you. You'll probably need a tech to come and do a leak test on your system.
Actually no technical skills needed to check the Freon level. Home central air conditioners have a Freon scale on it. One just need to read the scale and determine whether the level of Freon is sufficient.
blue
Depends on the size of the unit and when it was built. Modern A/C units contain no Freon (R12). It was banned in 1996. Call a home A/C professional.
5.1 Lbs
after twelve thousand years, so don't worry
No easy way of doing this. These units do not come with service valves.