If there is a legal self help group in your area, contact them. Google legal self help in your state and city. If you are lucky you live near a college or university that has students look at these types of issues for practice and it won't cost you anything for their help. They can get you started with a small claims court action. The Nolo Press books for legal help are especially good; I've included a link to the California Small Claims Guide for example.
Also try your local Better Business Bureau, State Attorney General's Office, Consumer Protection Agency.
Have ALL of your documentation organized and try not to get emotional when asking for help.
If small claims court isn't available to you, you'll need a lawyer that specializes in automotive repair issues & lemon law issues. But you may end up spending more in legal fees than you could possibly collect.
Everybody Guide to Small Claims Court in California Everybody Guide to... Better Business Bureau Start With Trust Auto Repair Basics Consumer Information Consumer Complaints and Protection