You take the ship shuttle to the Christopher Columbus Statute which is at the beginning of Las Ramblas Street. There is a kiosk where you can obtain information on the Turistic bus system. This is a great way to see Barcelona. You can also access their website and obtain a lot of information even before you leave home. Also, sit outside up on top if the weather permits.
DO NOT MISS the Sagrada Famililia Church and the Gothic Quarter.
Hope that this helps.