To read the temperature look for examples with the LM35 or TMP36 temp sensor. These go up to ~150C.
The simplest "UI" would be a potentiometer. If you want an LCD and buttons to control it then you should look up the Phi Shield or similar.
To control the temperature knob an RC servo would be the simplest method as it already has built-in feedback and has the torque you need. If you're trying to turn the knob on a stove or such then I'd really recommend getting an electric hot plate instead and using a Powerswitch Tail or similar to turn it on/off. Automating a gas control knob with any kind of motor is kinda spooky.