Houston can be challenging for gardeners that move to the area but the truth is that if you work with it you can grow something pretty much all year round. You just have to select the right plants for the area. The other truth is that quite often the soil in the area is not very good.
If I were to make one recommendation for successful gardening (vegetable gardening) it would be to used raised beds. Makes all the difference in the world.
A very good source for information specific to Houston is an organization called Urban Harvest www.urbanharvest.org which not only has excellent advise on their web site but also offers all kinds of classes at quite reasonable rates. The guy that used to be in charge of Urban Harvest, Bob Randall, has written an extremely good book called Year Round Fruit, Vebetables and Flowers for Metro Houston which has everything you need to know.
You have to remember that going from the north side of town to the south side of town can take you from 8A to at least 9 on the temperature scale, Interstate 10 is kind of a dividing line for both temperatue and soil.
Houston is very good for fall/winter gardens so you have an opportunity right now to get started with that. Brocoli, Cabbage, Cauliflower, etc. all grow very well in Houston during the cooler months. It's a little late to start fall tomatoes but you might give them a try. Later in the year would be a good time to plant citrus trees which do very well in Houston.
A couple of useful places for supplies, in addition to Buchanan's, are Southwest Fertilizer and Wabash Feed and Antiquities.