You can tell Glass to do lots of things like a Google search, take a picture, record a video, get directions, send a message, and make a call. All you have to do is say the magic words "ok glass" and then say your voice action aloud.
Glass must be activated and your display must be on in order to start a voice action. To get Glass to wake up and listen, tap the touchpad once or lift your head to the Head Wake angle (if you have Head Wake Up on).
Once you say "ok glass," you'll see all available voice actions. Scroll through this list by tilting your head up or down or swiping forward and back.
Continue through this guide to learn some more Glass voice actions.