You'd have to pay capital gains tax, which by holding it less than 1 year it would be short term capital gains, which is taxed at your regular tax rate. Long-term capital gains (if you held the property for 1 year and 1 day or longer) would be maximum of 15% tax rate, but is 5% for those in the 10 or 15% bracket (2008 the 5% rate becomes 0%). More than likely your short-term gain of 13,000 would be taxed at 15 or 25%. 15% bracket goes up to taxable income of $31,850, then it changes to 25%. So with salary of 28,600 and short-term gain of 13,000 you are at an AGI of $41,600 and depending whether you itemize or take standard deduction you might be just under or just over the 31,850 threshhold. You don't have to worry about state income tax, since Texas doesn't have one.

The other alternative if you didn't consider selling the house to be capital gains, would be to say that you bought and sold houses for a living, which would make all that be self-employment income which would result in more taxes for you then if it was capital gains income even as short-term gains. Self-employment income is subject to a tax of 15.3% of 92.35% of net self-employment income on top of the regular federal tax.