I'll fill in the blanks for you as far as a tub goes...
If you just have a tub (not in a rack) you'll need a UTH and a thermostat to control the temps of the UTH. You'll need a digital thermometer/hygrometer (Acurite=12 bucks). You'll need a water dish and 2 hides. Paper for the substrate is free (newspaper). I think that's it....
The initial outlay of money can be more than one might expect, but it makes total sense to purchase these items and have your tub temps and humidity stable prior to purchasing a snake.
Insofar as the purchase of a snake is concerned, I would strongly suggest purchasing through a breeder. Petco is a store to stay out of for a myriad of reasons; just do a search for Petco!!
Hope this gets you started!