If you have liability insurance, your insurnace company should be defending you. The first thing is therefore to make them aware of the lawsuit--call them up, send them a copy of the papers, etc.
If they refuse to defend you, check your policy again--if you were paying for liability coverage, you should be getting their defense. Make sure you were buying the coverage you thought you had.
(Also, remember your deductible--you need to pay up to the level of the deductible, if sued and you lose or settle.)