And if you don't want to pay the extra fee usually charged, immediately go to the counter and request it. Usually there are exit-row seats that nobody paid for and they'll give it to you.
A tip for flying Southwest (who doesn't assign seats): Don't be scared to hop into one of the exit row seats, they are fair game like all other seats. I notice that a lot of people will walk right by them (who are traveling alone and could obviously use the extra leg room) just because there is a flight attendant standing in the row. Just ask if you can slide in, they are just standing there to get out of the way waiting for the rows to fill up so they can give the exit-row speech.