First, you need to become an Annual Member, Monthly Member, or buy a Single Trip or Adventure Pass to be able to use Bluebikes. 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 Single Trip or Adventure Pass you'll also receive 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.