first time to Barcelona via cruise ship, Regal Princess.
Congratulations, you'll love Barcelona :)
Good for you. Barcelona is great. But what are you asking? Are you docked for three days? Returning to the ship to sleep/dine? Are you staying in Barcelona before or after the cruise? If so, what area? What do you like to do/not like to do? There are many people on the board who can help you enjoy Barcelona including some who live there. But you need to provide more information.
Do a city tour with Barcelona Day Tours. They are great. Also, visit Monserratt if you have time.