Shane,
At minumum you need a temperature sensor along with suitable signal conditioning circuitry and an A/D converter to get the temperature to your program. Similarly you need a power controller for the heater, probably a relay, and an output device from the computer to activate the power controller, possibly an amplifier or intermediate relay. The exact parameters for all these devices depend upon the temperature range, the power of the heater, the voltage source for the heater, how precisely and accurately you need to control the temprature, and how fast the temperature can change.
Also, the human interface and safety considerations have not been factored in.
After you have answers to all these questions, then it is time to start the design of your program. If you can provide specifics of your application, someone will probably be able to help.
Lynn
