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You will want to file both an "answer" to the complaint (usually a "general denial and affirmative defenses"), and a "counter complaint" (in which you sue them for (1) breach of contract and (2) negligence).
If you are in small claims court, this should be fairly straight forward - the court hears argument from both sides and allows you to present your arguments informally. I would still recommend getting a contractor or other building tradesman to act as an expert but it is not absolutely mandatory.
If you are in general civil court - expect the process to be more formal, and you will definitely want to have an expert on your side.
They can sue now, even though it has only been a short time (likewise, you can also sue, even though it has only been a short time).
You can get a lawyer if you want to (this really isn't a big enough claim to warrant a contingency fee - where the lawyer advances their services and costs of litigation for a portion of your recovery (even at 40%, if the damages are $5,000.00, the attorney is not going to be able to represent you for a possible recovery of $2,000.00). But if this matter is a small claims case, these are designed for you to represent yourself (without a lawyer).
If your damages are greater - you may want to look for a lawyer.
Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.