See a specialist. System needs to be purged, leak checked & Recharged.....
R134a
R134a.
No you don't have to change the A/C Compressor.
Yes!
You can't simply put R134a into an R12 system. You'd have to, at a minimum, change the hose connections and change the oil to PAG. the proper method to retro fit involves adding new charge port adapters, changing the oil in the compressor, and replacing the drier/accumulator. I know a lot of mechanics who do nothing more than add the port adapters and shoot in some extra pag oil.
The 134a plus just has PAG oil added already, so that you (theoretically) don't have to add it after adding the R134a.
A 2001uses R134a refrigerant.
It compresses the refrigerant (probably r134a) and pushes it through the ac system.
You are low on refrigerant, google R134a...
too much oil in system or compressor not working properly are some possibilities
PAG, the same as any other R134a air conditioning system.
Start adding, once you get enough pressure to be over the safety low pressure switch it will come on so you can fill it completely. The switch is put in there to prevent the compressor from coming on when no refrigerant is presesnt and ruining the unit. You might want to find out why you are so low on freon before filling it up again.