I've been to a lot of auto repair shops here on Southern California. A few things I learned are:
1) Ask if it's necessary to do the repair now, or can it wait. If it can wait, then take it to a different shop or ask around for a second opinion. If the car isn't going to explode or fall apart, I would ask/call around first.
2) Usually it is best to stick with OEM parts when getting your car worked on. Ask the shop if they use OEM or aftermarket. The better ones know from experience that it's best to use OEM. Some things, not a lot, like brakes can be better with aftermarket parts.
3) Take someone with you, preferably a guy, if you're a girl. For some reason, the mechanics or technicians seem to think every guy out there knows about cars. Plus it's better to have a second set of brains to help sort the jargon out.
4) Get a receipt for the repairs there were completed. Also, have them write estimates for "recommended repairs."
5) Make sure you have good communication. You don't want any surprise repairs when you pick your car up later or next day. They should be calling you for permission for anything beyond the initial repair order.