What's the Tresping Law in Texas?

I live in Texas and I keep hearing that you can shoot someone if they're tresping on your property. I've heard this from the guys who had built my fence for me. My husband and I have been having trouble with different people driving up in our driveway and getting out asking us for money, since we're the only two story house in the neighborhood. We've also had people coming onto our property trying to cut our gr without our permission. Neighbors and different people cutting across our yard for the hell of it, then saying "Oh, I didn't know someone was living here". There are two vehicles parked in our carport that belongs to my husband and I. Also we'll get people ringing our door bell asking if we have a job for them. We've also had people drive over our property for the hell of it. We've had our PRIVATE PROPERTY NO TRESPING, KEEP OUT, BEWARE OF DOG, AND KEEP OFF GR signs up for months every since we bought our new home. We put up the fence since we have been having trouble, but it still isn't enough for the idiots to keep away from our private property. I don't know if it's just our neighborhood of idiots or people are just too lazy to read. The signs don't seem to work at all. People seem like they're either dumb, illiterate, or just rude. It's not hard to read a simple sign and get a clue not to tresp. I was in Police Academy, but my Texas Criminal and Traffic Law Manual is out of date (2007 to 2008). I'm sure by now that the laws have changed, since they change every two years. Anyway, if anyone has an answer or a suggestion, please let me know I would appreciate it.