If you're doing this as a first A/C project I recommend this book.
http://www.acsource.com/index.asp?PageAction=VIEWPROD&ProdID=8
From Mastercool. Covers all you need to know about R-12 v R134a. High low pressure targets. (They are different). Components, troubleshooting, how, when, where. to add oil. and so on...high/low switches. Fan controls...