Southwest doesn't assign seats when you buy a ticket. When you check in for your flight, either at the counter, kiosk, curbside check in, or online, you will be issued a boarding pass.
That boarding pass will have either an A, B, or C on it. When you get to the gate - you will see three signs, for lines of people, marked A, B, C. There is also an area for pre-boards, for people with disabilities or children 4 and under.
When the plane is ready to board - preboards will go first. Then the people in the A line will board, then B, then C. If it is an originating flight (i.e., empty), then the preboards, and the A passengers have first opportunity to choose their seats - they are pretty much guaranteed the opportunity to sit whereever they choose. Then the "B" passes are allowed to board, and finally the "C" passes.
If a plane is a continuing flight - some, or most, of the seats may already be taken, so you may have a hard time finding seats together if you have a larger party size.
The earlier you check in - the better your chances for that "A" boarding pass.