Read through this thread, it covers all of the basics when it comes to cage heating/measuring temperatures/humidity. It also covers a few different options you will have.
http://ball-pythons.net/forums/showt...t-Thermometers
When using a heating pad you MUST control the heating pad with a thermostat or lamp dimmer. Stay away from the junk sold in pet stores. The thread I linked to above has recommended thermostats at all price points.
Heating pads also require you to use a probed thermometer as they only heat surfaces, and not the air in the cage. once again the thread I linked to above has a recommended product.
Lastly heating pads require a thin substrate layer. 1/2" thick or less.
Here is another good thread to read, it covers basic care and husbandry:
http://ball-pythons.net/forums/showt...ius)-Caresheet