You can get your tax id number
here I'd get that first since you will need it to open a business checking account, then get your dba at the county courthouse, they usuallu give you the paper work the same day so you will be waiting on the state to get your ID number to you.
But before doing either of those I would find a CPA to work with, they can walk you through the steps and will go along way in making sure you get everything, including your books set up properly the first time.
Speaking from experience I'd invest my first dollars with the CPA. Its amazing how many things you will need to get in place and maintain to keep yourself out of trouble and any money spent with a CPA will be worth it in the long run.
Good Luck,