Did you do any research on the SW official site before booking? they have everything you need to know about boarding and seating.
Basically....you don't get to choose a seat until you are on the plane. So....you have two choices for checkin. You can do nothing addtl and then be online 24 hrs prior to your departure. If your flight is at 8am on a Sat, you need to be online by 8am, Fri, to get checked in. After you checkin, you will get your boarding pass. It will list a letter (A, B or C) and within that letter, a number. When you get to the gate and they announce boarding, you will get into the line up according to your letter/number. The A group will start with A16 because those paying full fare or business select get the A1-15 slots. Then, the A group will board. Followed by 'family boarding' which means that families with kids under 4 are allowed to board here, no matter what slot they got. Then the B group boards, followed by the poor C group. Those in the A and B groups usually have no issue getting pairs of seats together.
Or, you can buy Early Bird Checkin for about $12 or so pp, each way. This will get you checked in at the 36 hr window! You do nothing, SW does it automatically...which is especially nice when headed home! At the 24 hr window, you can go online and print out the boarding passes and find out what slot you got.
You might want to go to the SW site and look around. there is a 'boarding school' area that will help you understand what will happen.