How do I answer a complaint for modification of child custody?

There really isn't a lot in terms of online forms that will be helpful.  You'll need to prepare an affidavit in a form similar to the one your husband filed, with numbered paragraphs that reply to the factual claims he's made, and that state your own "counter-" facts.  If there really hasn't been any change in the situation in the year and a half since the divorce (or at least, nothing that wasn't anticipated by both sides at the time of the divorce), you should also make that clear.

If you have been in Texas for at least six months, and he has filed in South Carolina, your first line of defense is that this now has to be done in Texas as the children's home state, under the Uniform Child Custody Jurisdiction & Enforcement Act ("UCCJEA").

You really should have legal representation on this; much is at stake here.  If money is an issue, see if you qualify for Legal Aid or see if they can recommend someone to help you.  Also, check if there is a law school nearby to where you live; they typically run free/low cost clinics that handle these type cases.  Contact the local Bar Association in your county; they may have a list of attorneys who will take your case "pro bono" (for free) or at least for a reduced fee based on your income/circumstances.  Additionally, google women's support groups in your area, they can usually provide a list of attorneys who can help. Finally, contact your state's Department of Social Services, they may be able to refer you to free legal services.