The DBA can be filed at the downtown civil courts building on the 3rd floor or at any county court annex. It's $15 if you have the form notarized in advance, and $16 if you don't.
http://www.cclerk.hctx.net/PersonalRec/assumnam.htm
The sales and use tax permit can be filed online. Just go here:
http://www.window.state.tx.us/taxpermit/
One thing I would recommend is getting a federal employer identification number (FEIN) from the IRS, even if you are a sole proprietor and don't have employees. You can still file business taxes on your personal tax return, but if you ever have to fill out a W-9 for omeone or you hire contract (1099) employees, it keeps you from having to give your social security number to them.