I had just read a story about a guy who used R-22 (home and commercial building refrigerant) in his car. The compressor seized, broke the fan belt, car overheated, and blew a head gasket. I'm not being politically correct here, this is a safety issue.
The conversion can be done one of three ways:
1) If your system is empty, remove the fitting that allows you to add R-12 and install the R-134a fitting. The fitting is about $11.50 here:
A/C Adaptor Kits & Fittings
Recharge the system with the correct amount of R-134a.
2) Have the system professionally ($) evacuated, replace the receiver-dryer, change the fittings as above, re-evacuate the system, replace the 4 oz. of compressor mineral oil with R-134a compatible PAG-46 oil, and recharge the system with the correct amount of R134a, about 30 oz.
3) Buy the kit from our sponsor, P/N 10-7504-128-M207. Then recharge the system as above after a vacuum evacuation.
Good Luck,
Mark