With Southwest you can check-in no earlier than 24 hours before your flight. You can purchase Early Bird check-in for $12.50 per person per flight. When you check-in you just get a boarding group; A, B, or C with a number. The sooner you check-in, the more likely you are to get an A or B group. There are no assigned seats on SWA. It's first come first served. If you are travleing with a group and sitting together is a priority, then getting Early Bird Check-in is the way to go. Especially if you are traveling with children. We never buy Early Bird. I just arrange to be ready to check-in at 24 hours prior. You can check-in on your smartphone with their app. I almost always an A or a low number B. It's worth it for the low prices SWA has on tickets and no bag fees. Good Luck!
Edited to add: If you are using DME and doing resort check-in for your return flight, they always did the check-in for us and always got us in A group.