If you want to use
Disney Cruise Line transportation (ground transfers), you have two options. The timings are based on pre-covid information and subject to change.
(1) From the airport to the port - this takes you from MCO airport directly to the port on the day of the cruise. They start running them at about 9 am and a new bus leaves as it is full. The lineup for the buses usually starts 8 am or earlier, so if you want to be on the first bus or two, get there early. For Disney transportation this is the option that will get you to the port the earliest.
(2) From a WDW resort hotel to the port - this takes you from a WDW resort hotel directly to the port on the day of the cruise. These will depart in a window between about 11 am and 1:30 pm - you don't get to pick the time,
DCL does. The night before Disney will leave a letter under your room door letting you know when you will be picked up, where to meet for the pick up, and when your luggage will be picked up from your room with instructions on where to leave it. This bus will usually get you to the port after general boarding has been announced, once the bulk of the huge initial crowd is on the ship.
If you browse or search the cruise subforum there are lots of posts about ground transfers as well as other options, and about hotels near the port if you want to explore that option:
https://www.disboards.com/forums/disney-cruise-line-forum.9/