Review the Lawsuit
Make sure you understand the claims against you. Are other people you know involved? If so, contact them.
When Do I Have to Respond?
Determine when you have to Answer the Complaint (or "respond"). Usually it is 30 days. If you are late already, call and try to get an extension to respond (or, if you hire a lawyer, they can handle it).
Gather All Documentation and Evidence
After reviewing the complaint, figure out and gather all evidence you need to present to a lawyer for their review or for you to defend the case. If photos need to be taken, do it as soon as possible so you don't lose the opportunity. If there are witnesses, obtain their contact information for future use.
Is Insurance Involved?
Determine if insurance may be involved. Like, if in an auto accident, you would contact your insurance company and they will usually provide a defense for you.
Contact an attorney?
You should at least talk to one or two attorneys. Even if you think the claim lacks merit, you may be missing something. So, just be safe by consulting with a professional.