Try to earn ACT scores that guarantee your admission. Texas State allows you to submit either SAT or ACT scores. An 18 in English, a 21 in Reading, a 22 in Mathematics and a 24 in Science can earn you a spot at Texas State.
Keep your grade point average as high as possible. Try to maintain all A's if possible while completing either the Recommended or Distinguished High School Program. Texas State's website provides a list of the credits necessary to complete these programs. Set a target grade point average for yourself and use an online calculator to find out how long you will need to maintain all A's to reach it.
Emphasize your participation in extracurricular activities on your application. Mention any volunteering and work experience you have as well. Texas State, like most universities, values your SAT scores, but those numbers only tell the school how you perform on a standardized test under certain stressful conditions. It is important for you to be well-rounded. Someone who earns a 1600 on the SAT but has never played a varsity sport or volunteered with her youth group is not necessarily more likely to succeed in college.
Give the school a sense of your personality. Show them that there's more to you than your standardized test scores. If Texas State asks you to come for an admissions interview, spend time preparing. Ask a parent or friend to help practice. Dress professionally for the interview and show up early. Put as much time as you can into your admissions essay -- show the admissions office why you would make a valuable addition to the Texas State student body.