Never lived in Texas, so can not tell for sure, but what I heard is you can get decent private room for about $300 and If you keep $150 for food stuffs(might be less, if you work in resturant and can take some meals there and make your own meal otherwise).......... $50 for Auto Insurance ( As you are going to buy cheap used car, you might just want to keep liability insurance), $60 for gas ( If you dont have to drive that much), Utilities should not be that much, $40 for cell phone ( I think you can easily ditch data for now).

$100 for other miscellinious,

Your montly expenses should stay below $700 ( of course i did not count any of your school related expenses). Now you can do math based on your monthly earning so far or comparing that of your other friends. I dont see much room to play around though. And old cars can break at any time so, try to keep some money handy in case of emergency, if you can)
And If you have SSN, why dont you try some Credit cards for now....... you might get some Creditline, even if it is $2,000 with 0% introductory APR for about a year...... That will give you some room to play around and dont need to worry about interest at least for a while.
Again Good Luck.