The simplest way to prove how long you've lived here is to sign an affidavit that states when you moved to Texas. That's good enough for most situations.
However, if anyone decided to contest the date you state in the affidavit, then the court would need to see other evidence to back that date up.
There are several things you can use as acceptable evidence. Leases or utility bills are commonly used. You could also use mail that has been addressed to you with a postmark that indicated when you started receiving mail here.