In most states including Texas you are eligible to apply for a license after receiving approval from the National Registry of Emergency Medical Technicians (NREMT).
Here are the steps
Turn 18 years of age
Graduate High School or obtain a GED or homeschool equivalent
Attend a school approved by the Texas Department of Health Services (DSHS) and obtain a Certificate of Completion
Pass the NREMT Psychomotor Exam
Pass the NREMT Cognitive Exam
Apply for a Texas state license