You have twenty days and then a Monday to respond. In Texas, answers are always due on Monday following twenty days after service. It sounds like you were also served with Requests For Admissions, but those aren't due for 50 days after service.
If you don't file an answer, a default judgment will be entered, and then collection begins. As suggested by Debt Guy, that's not the easiest thing to do in Texas. However, you still need to avoid a judgment.
First, file an answer to the lawsuit. You don't even need to go to court to do this - it can be done through the mail (there's lots of advice about how to do that in this forum). It can be very simple: I deny the debt. Be sure and send a copy, certified, to the lawyer for the Plaintiff.
Next, do some research - find your bills and figure out when you last made a payment. Then you'll know if the statute of limitations applies.
Remember to file Responses to the Admissions if you haven't settled before the 50 day mark - you don't want to let those go. If you do, they are deemed to be "admitted", which means its the same as if you answered "yes" to each question.
In the meanwhile, sit down and take a hard look at your finances. If you owe this money, figure out what a reasonable payment plan might be and make an offer to the Plaintiff. Again, there's lots of advice on this forum about how to negotiate a settlement.
No need to panic! Come back for more help as you need it.
And congratulations on the new Texan - there are never enough of them!