Who owns the restaurant? Your lawsuit won't get very far if you're not suing the right party. Identify who the "usual suspects" are in your case. For example, your target may be a specific individual, a local franchisee, and/or a national chain.
How seriously were you injured? Make sure your injury is serious enough to warrant a lawsuit. If not, you run the risk of having your case deemed a frivolous lawsuit and tossed out of court. One of your first steps should be to write down everything that has happened to you since the incident with the restaurant. This may include things like medical bills, hospital visits, any lost work or wages, etc.
Did you seek immediate medical attention? If you fell ill or were hurt at the restaurant, then you should head to the doctor's office soon to get a professional opinion on your health. The doctor's comments on your injuries will carry more evidentiary weight than your own in most cases.
For extra guidance on whether you have a "good case," you may want to consult an experienced personal injury attorney near you.