Get to the bus stop a little before 6 AM.
Buses start about an hour before park opening or at about 6:30 AM whichever comes first.
These early buses don't run as often as buses run mid-day.
Don't try to cut it too close. Otherwise you might find the first bus you see too crowded to get onto and the next bus too late to make rope drop (and get a boarding group for Galaxy whatever).