What to do if I just found out that I have a theft charge pending against me?

In Texas, a theft charge can be filed in a JP court, a county court, or a district court, depending on the level of the offense.  Regardless of where the case is filed, it will be handled by the DA's office in the county where the charge is pending.  So... as a starting point, contact the DA's office and see if they have the case yet.  If they have the case, ask for a "discovery packet."  Some DA's office have open file policies while others make you wait until your first court appearance-- but this packet will tell you who filed the theft charges.  You may also be able to pay restitution and have the charges dismissed without a formal case being filed against you if a prosecutor is willing to talk to you.

Another way to find out information is to go to the county website where you think the charges are pending and run a search on yourself.  More and more counties are offering online systems.  Most of these systems are also free and will give additional info, like warrant information.

A final option is to run a criminal history on yourself through the Texas DPS website.  Depending on whether or not the county is up to date on their reporting requirements, the info on the new charge could be on there as well and tell you exactly which law enforcement agency is filing charges against you.

The DA's office does not have to talk to you.  But.. any information filed with the clerks of the JP, County Court, or District Court is freely accessible by anyone.  If you want to get ahead of the charges, you may want to considering hiring an attorney that practices in your area just to find these details out for you.  Often paying restitution for a dismissal is much cheaper than paying for a bond while your charges are pending.