Frank, mechanics get years of education and experience to be able to diagnose these types of problems. If you really want to do this yourself, get a repair manual, there are some good online sources (alldatadiy.com) and get a scanner. Read the codes and then do your homework on what can and cannot cause the codes to be set. We have some great articles in our Encyclopedia about common fault codes: http://repairpal.com/encyclopedia
This is one of the most difficult types of diagnosis on your car, so be prepared and do your homework. Post the codes here if you need some guidance. Good Luck!