One of the easiest ways to implement a temperature sensor on the Galileo (I'm using the Gen 1 board) is to use an LM35DZ. This inexpensive chip delivers an analog voltage proportional to the temperature and is easily handled by one of the ADC lines (in this example A1). It comes calibrated to deliver 10 millivolts per degree [C] - about as simple as it gets. Figure 1 is the schematic. Just those three connections gets you the temperature within a range of +2 C - +150 C.
The short sketch attached has been written using verbose variable names, is straightforward, and a sample screen shot is below.