The first thing you should do is educate yourself about the various foundation repair methods. An educated consumer is a smart consumer who is less likely to be ripped off by an unscrupulous, fly-by-night contractor.
Make sure the contractor you hire is both licensed and insured, and don’t be swayed by lowball estimates. You also want to make sure the repair comes with a warranty. However, remember that the warranty is only as good as the company offering it. Finally, ask if the cost of the repair includes things like permits and engineering costs. You don’t want any surprises.