Hatchlings are roughly 10-12 inches long, and eat pinkies (newborn mice) every 4 to 7 days. As the hatchlings grow, the number of food items is increased, and when the hatchling is big enough, they move up to fuzzies/fluffs (baby mice that have just started to grow fur), and so on until the snake is ready for small adult mice, usually at about 12 to 18 months in age. A good rule to follow is that the food item should not be any bigger than 1.5 times the size of your snake's belly at its widest point.
Adults are between 3.5 to 6 feet in length (averaging about 4'), and large specimens may weigh a couple of pounds! Adults are fed between 7 to 10 days apart, and generally eat one or two appropriately sized rodents. IMPORTANT: Do not handle a snake after it has been fed, leave it 2-3 days or until you see *snake pancakes*! 
Rodents should be thawed and warmed before they are fed to the snake....never refreeze uneaten prey items.
PLEASE....NEVER FEED LIVE RODENTS TO YOUR SNAKES, IT IS ILLEGAL AND POTENTIALLY HAZARDOUS TO YOUR PET.
Vivarium size required at adulthood: 36" x 18" x 18" (hatchlings prefer a smaller, and more secure feeling accommodation until they are roughly 6 months old)
Heating: Heatmat and thermostat, or use a red reflector bulb (red light is not visible to cornsnakes and therefore causes less disturbance) and dimmer stat. Temperature should be 82-86 degrees fahrenheit at one end, and 72-76 degrees fahrenheit at the other end. This *warm end* and *cool end* setup provides a healthy temperature gradient in which snakes can thrive well.and digest meals safely. Thermometers should be placed at both ends of the vivarium to ensure that the temperature is correctly set.
Note: Cornsnakes do not require light, as they are most active at dawn and dusk (diurnal), so no need for UV tubes etc.
Substrate: Newspaper is quick and easy to change, and adds hygiene to convenience as a positive point for its use in day-to-day husbandry practices.
Cleaning: Daily....Remove soiled newspaper as required, and wipe the inside of the vivarium with a suitable reptile-safe disinfectant, and provide fresh water. Once a month, wash the vivarium from top to bottom with a dilute bleach solution (remove the snakes first!), rinse very thoroughly, allow to dry and air, then replace the paper and furnishings...and the snakes! 
You will also require, a water bowl, and appropriate sized hides....it's always worth having more than one hide so that they can be positioned in various temperature ranges within the vivarium.
Please note, that this is a very abbreviated caresheet, and is intended to provide a generalised outline of care requirements that your future pet will have... Full support is given to assist beginners in making educated decisions about the care and welfare of their chosen animal. To learn more, please feel free to contact me, I'll be happy to help.