You will need a hunting license... if you just moved to TX you may not qualify for a resident license yet. I think that takes 6 months. If you plan to keep a collection of native herps (bad me, I forgot the #s that qualify), or sell even one of them you will need another permit for that.
Hunting from your car.. It is much better than it used to be, there is more of a consensus to allow it. But the water around that is still muddy if you ask me. There is an email, I think Chris McMartin still has it hosted on his site that is from the head of TP&W agreeing that such enforcement was not the spirit of the law. His page is a wonderful resource and I hope it is OK for me to link you to it..
All in all, though not much has changed, you just go, have a good time, try to be safe and not freak out the locals too much...
This time of year you might want to look into the more Western locals for Alterna.... North Brewster Co, Jeff Davis Co, Hudspeth Co, this is part of their good times while Val Verde and Terrell Co have sort of petered out.
McMartinVille Field Herping Laws and Equipment