A hatchling is better off in a smaller tank to start with and put into a three foot tank when they're bigger. They get stressed by a lot of open space, so if you're forced to use it, make sure you have all of the floor covered with hides, plastic foliage etc.
I wouldn't recommend it though. If it has sliding glass doors, a hatchling wouldn't have too much trouble squeezing between the panes of glass to escape. Not to mention that you'll keep thinking it's escaped evrey few minutes, because in a 3 foot viv, a hatchling could hide for weeks without you seeing it!
Aspen is fine as a substrate.
The best idea is to clean the poop/wee as it happens (known as spot-cleaning) and then every month or so empty out the whole viv, disinfect or replace all the hides/plastic foliage/water bowl, disinfect the viv and replace all of the substrate. Change the water every couple of days unless it gets pooped in (change immediately!).
Baby Corns are quite fast and can be a bit nippy, but they mostly calm down as they get used to you and become confident that you won't hurt them. They make the most fuss when you first go to pick them up, but you usually find they calm down a bit once you have hold of them.
Depend on the shop as to how you buy the snake and all the equipment. Some do very good deals on "starter kits", but don't let them tell you that a thermostat is optional. It's very important to control whatever heating device you decide on.
The best reference book is "Corn Snakes - A Comprehensive Owner's Guide" by Kathy and Bill Love. Loads of photos as well, so you can start window shopping for your next Corn!
Sure someone will be along soon with a list of links to FAQs for beginners.
Welcome to the world's most addictive hobby!