I'm looking at buying a gun for self defense...?

I live in the state of Texas and the law for firearms is that you have to be atleast 18 to purchase a rifle/shotgun including ault weapons or 21 for a pistol....my first choice would be a pistol but I'm only 19...before you jump to any conclusions thinking that I'm a young delinquent trying to cause trouble....I live with only my mom so I'm pretty much the man of the house...We've had a lot of problems with some of her boyfriends...we've called the cops on them...I've fought them off...we've got restraining orders but now they've moved to vandalizing our things while we aren't at the house...I feel as if this is the last resort to finally feel some comfort in living here....so basicly all I ask of you is if the State law is reinforced by all stores? There are only a few places to get some which are just mostly for hunting purposes only...I need something for self defense? Any suggestions? What and where to buy...anything would help at this point...thanks...