So the top of your ceiling is 20 feet up there? Don't try to do it all with a ladder. First off, it's harder. Second, you could die. Use a pole.
Roll it on to a good tune - like rolling down the river, or let the good times roll. Get a good roller frame too. That's sold separately
Painting a flat wall is pretty easy. It's the corners that are a massive pain. A good corner brush makes that much easier