Substrate is what covers the tank floor. Most people use aspen bedding (do not use pine or cedar), cypress bedding, newspaper, paper towel, repticarpet, or Eco earth. Whatever you use, it shouldn't be more than an inch thick.
unplug your heat mat until you can buy a thermostat. However, you really need a lamp dimmer ($10 at Home Depot) and a thermometer to measure the hot spot ($10 at Walmart). The dimmer can control the power to the lamp and thus helping to raise the temp or lower it.
once you get paid, invest in a second thermometer, a thermostat, and a ball python care book (Petsmart has one for $8 I think). You should also read the care sheet on this forum.it will help you a lot!