You are in the husbandry section of this site, if you look at the very first topic - Husbandry & Basic Care FAQ you will find all the information you need.
I am posting one section of it to partially answer your question but please go read the entire thread. Be patient, take a few hours to read a few things and get the basics down before you jump into it.
I may also suggest that you please, please, please do not rely on a pet store for your information. Most know nothing really and have had very little experience with snake care other than letting you know what they sell, which may not be what you snake needs at all.
My suggestion on a tank would be the Sterilite containers mentioned in the thread I was talking about. They are very inexpensive and you would be surprised how many folks use them for cages. They have secure locking lids but I would drill a hole and install locks as well, because snakes are infamous escape artists.
Before you buy, make sure you read about heating, heat gradients, feeding, shedding, hides, proper substrate, mites and other illnesses to check for when considering purchasing a snake, thermometers, thermostats, heating devices, and any other items you will need when bringing your snake home.
It is best to buy everything before hand so you can clean, sterilize and set it up. Additionally it will give you the time needed to allow the tank & substrate to heat to the right temperature so you make the transition as seemless as possible. Don't try to do it all in one day because the tank needs time to air after sterilizing.
If you are concerned about losing the snake you want, ask them if you can put a deposit on it, I have never yet been to one store where they would not hold it under that circumstance but there may be one that wants you to pay it in full before they will hold it. Either way you would have the little fella or gal you want but giving yourself time to take care of all you need to before bringing him/her home.
Anyway here is the partial info from the post I talked about. Good luck.
Joejr14 said:
Husbandry/Basic Care FAQ:
Q: What can I use for a cage?
A: There are a wide variety of choices. Most 'breeders' use some form of sterilite boxes/tubs, but glass aquariums are perfectly fine. Homemade cages are also fine, provided that wood is sealed and the cage is secure proof.
*A note on the cage being secure. Every single snake owner that has any decent amount of snakes has had snakes escape. And I do mean everyone, including breeders. Having a great secure cage is worthless if you do not take the time to make sure your cage is locked, shut, and tight. Snakes in general are escape experts and will get out of a cage if there is the slightest space/hole anywhere.
Q: How big does my cage need to be?
A: Most adult cornsnakes can be housed in 20 gallon long aquariums. All-Glass makes 'Critter Cages' that have a sliding lid with back latches that work great. If you choose to go the sterilite route, adults can be housed in 28qt sweater boxes and hatchlings through sub-adults in boxes ranging from 6qts to 12qts.
