Here are the temps and humidity that ball pythons require:
Cool side: 78-80 F (nothing under 75)
Warm side: 81-83 F
Hot spot: 90-92 F
Humidity: 50-60%, 70% during shed
The best way to heat it is combining a heat lamp w/ dimmer on top of the 20 gal, and then a UTH on one side under a hide, to create the hot spot. A good brand of UTH are zoo med and ultratherm. Your UTH will NEED a thermostat to control the temps, check out Vivarium Electronics or Herpstat, otherwise your snake will be susceptible to burns. The heat lamp should use either an infrared lamp (50 watt is sufficient) or a CHE (ceramic heat emitter), but they both drain humidity a lot, so only use it if your house is cold (room temperature of under 75).
I suggest cypress mulch bedding (Zoo Med Forest Floor Cypress Mulch) or coco-husk for bedding, as humidity tends to suck in glass tank enclosures. Other beddings that work are aspen, paper towels or newspaper. You will need two identical-looking hides on both sides of the glass tank so the snake can thermoregulate, aka choose if he wants to be hot or cold. I highly suggest you downgrade to a 10 gal or a tub setup for now, because a 20 gal is much too big for a baby bp.
Take a look at the care sheet in the Ball Python Husbandry section. It will answer a lot of your questions.