How to file a written response to a complaint?

You file in the court clerk's office (the same court the complaint was filed in) a written response to the complaint. You admit anything which is inarguable--e.g. say that he got your name and address right; you would admit that. You deny anything that may or may not be true; by denying it, you will force him to prove it. You can also put in any "affirmative defenses," or reasons why he should not be able to prevail, such as if it's past the statute of limitations for this type of claim, or his injuries were due to his own neglience or carelessness, or if his losses were because he breached the contract first, entiting you to then consider it terminated--there are *many* potential affirmative defenses, depending on the facts and the type of claim(s) he is bringing. You can also put forth a counterclaim, if there is some reason he may owe you money. 
If the amount you are being sued for is less than the limit for small claims court in your state, it makes sense for you to answer this yourself. You should be able to get sample forms from your court, either in person or online, that could help you draft your answer. Once it is drafted and filed, you will also serve it on the plaintiff; the court clerk can advise you as to filing and serving.
If the amount you are being sued for is signficantly more than the small claims limit, it would make sense to retain an attorney and let the attorney--who knows the court rules and law and how to answer--do this for you.