20L's arent' usually recommended for hatchlings because they may not feel secure with all that space to be 'exposed' in- a 10L would be better at first. However, if you think you'd prefer to set it up in a 20L, it's fine as long as you provide PLENTY of hides. Also, it's recommended that you get a sliding-lock lid to easily prevent escapes.
For the bare basics you'll need a waterdish it can fully submerge itself in, two hides, a UTH, Thermometer/hygrometer, and a thermostat or rheostat.
Like others have said, lights aren't really necessary for a corn. I use a Zoomed 500R Thermostat and a Zoomed heat pad, and it's been working great. The heat pad will have a power cord that you should plug into one of the slots in the thermostat (like an extension cord), and that will control the heat. You may not want to stick the heat pad directly on the tank if you've revealed the sticky side, simply because it makes it extremely hard to take off. Rheostats are alright, I personally prefer the Thermostat because if the room temperatures change, you have to change the setting on the Rheostat. Either should come with a probe that you can stick inside or under the viv. I have mine outside on the heat pad so I don't have to deal with getting any wires under the lid of the tank.
The thermometer should have a probe to be placed on the glass (under the substrate) inside the tank on the hot side, since that will be the hottest part and you don't want your snake burned. Many probe thermometers come with a hygrometer (measures humidity). If you get one with that included, it will probably have a second probe that should be placed on the glass walling above the substrate so it can accurately measure ambient humidity. Again, for lack of wanting wires comming out of the tank, I keep my displayer inside the tank as well, away from the water.
The two hides can be anything you want. To an extent.

Again, as other have said, it should be the appropriate size for your snake to feel secure. If you have something like a long log or some other tunnel-like thing that stretched over both the hot and cool sides, you can use that too instead of having two hides. The main point is to give your snake the option of being hot or warm without sacrificing security. If your going to use ANYTHING from outside in your tank it should be baked and/or bleached first. (AKA: Don't just grab a log out in the wooks and stick it in your snakes tank. That's not cool.)
The water dish doesn't have many requirements, just that it's not so deep that the snake can't get out of it easily once in.
Anyways, I hope this was helpful to you despite a lot of it being restated. Good luck!