Usually, you would send a certified letter (with return receipt requested) to your vehicle's manufacturer. If you do not know the address, then it could be found in your warranty booklet. In the letter, be specific as to the problems that you have been experiencing with the vehicle.
After a reasonable period of time and the manufacturer fails to address your concerns, then you may file a claim with the Texas Department of Transportation. Keep in mind that you must do this within the first 24,000 miles or 24 months (whichever occurs first) -- or else you may have missed the deadline to assert your claim.
Even if you missed the deadline, other laws may be available to you. Feel free to call my office for a free case review. http://www.texaslemon.com