First time flying with Southwest.

As mentioned in the title. This will be my first time flying with Southwest. I purchased flight tickets before baggage fees went into effect. So luckily I won’t have to pay for my luggage. I’m curious on the seating though. Will I be given a pre assigned seat when doing check in, or is it pretty much first come first, & I sit wherever I want? I don’t want to sit in a seat someone else payed for.. What else can I expect from flying with Southwest.