First, you need to become an Annual Member, Monthly Member, or buy a 24-Hour or 72-Hour Pass to be able to use Hubway. Then, find available bikes on the System Map. If you're a member, you can insert your bike key into a dock to unlock a bike; or if you haven't received your bike key yet, you can use your credit card at the kiosk to get a temporary ride code (don't worry, you won't be charged again). If you purchase a 24-Hour Pass or 72-Hour Pass from a kiosk, you'll get a ride code, which you'll type into the keypad on the dock. Or you can choose to buy a bike key from a station with a key dispenser.
Once you've successfully inserted your member bike key or entered a ride code, the light on the dock will turn green. This means the bike is unlocked. To remove it from the dock, pull the bike out by the handlebars or lift the bike by the seat to release it.
For an introductory video to Hubway and more information on docking and undocking a bike, see the video that Harvard put together.