Texas is a "Right to work" state. What that really means is that your employer can fire you at any time for any reason. If they do, you are eligible for unemployment benefits.
If you truly believe that they are trying to fire you, make sure that you are always on time and do your job. This will make it harder for them to find things to write you up. I would additionally begin looking for another job.
If they write you up improperly, write a rebuttal to the warning. If your employer discriminates against you for your race, color, gender, age, religion, national origin, pregnancy, disability or veteran status; you may have a wrongful termination claim and contact an attorney.