You know how to send signals to the outside world. You might also want to receive input signals from the outside world. These signals can be divided into two types: digital signals and analog signals.
There are pins on the GPIO connectors of the Galileo than can be used to sense digital input; that is, an input that will be interpreted as either a 0 or 1 by a program. These are labelled on the Galileo. An example of how to use these is a simple button press circuit. To create this circuit, you'll need one of the 220-ohm resistors you used in the LED circuit. You'll also need a simple, single-pole, single-throw switch; you can get these at most electronics stores. You'll also need some jumper wires to connect the Galileo to your circuit. Here is the circuit diagram:
Now you'll need some code. You can use one of the example programs included with the IDE. Select the Examples | 02.Digital | Button code, like...