When you check in (you can check in online 24 hours prior to departure), you get a boarding pass with either an A, B or C on it. You board the plane in three groups starting with the letter A. When you get on the plane sit in any seat that isn't occupied. That's all there is to it.

If you check in online early, - 24 hours before your flight odds are you will get an A boarding pass. If you don't arrive at the airport early and check in at the airport you might get a C boarding pass, and you will be in the last group to board, and your seat selection will probably be mostly middle seats, especially if the plane is full.