About 2 weeks before your cruise you will receive a notification advising of your boarding time. Although the email will politely warn not to arrive early, as you will be turned away, most people ignore that and no one is turned away.
On our last cruise, we arrived at the port at 1200, were checked in straight away, and directed to a waiting room which already contained a large number of people. Boarding commenced at 1230 and we were all on board by 1245.
Enjoy your cruise - Steve