SW has no assigned seats. They have a boarding order that's roughly determined by the order you get your boarding passes.
They may have changed it slightly from when we went, but in general your pass has a number: A 1-60, B 1-60, C 1-whatever. You can purchase early bird in which SW puts in your order automatically for you at I believe 36 hours before the fight. Business class is either right before that or after. The next is people who check in after the 24 hour mark right before your flight (in order of check-in, so the earlier the better your number.)
There's some special cases. Handicapped guests can get pre boarding passes for themselves and a guest and board before the A group. If you have a very small child (not sure of age) they let you board I believe after the A group so you aren't separated.
It actually works really fast. They call pre boarding, then As, (small children), Bs, CS. They're actually numbered "pillars" that you stand by in order and then board.
First time we flew SW we didn't do early bird and I kind if stressed about it. Then POP gave us two boarding passes for DD and none for me. (Our names are similar.) We didn't realize it until we were going through security; SW was helpful, but it was a mess. Last trip we did early bird on the return trip so I wouldn't have to worry about it. For me it was worth it.
