If you buy the cheapest ticket, "economy" you will only be able to select a seat at check-in. You can do it online within 24 hours. My recommendation is to check-in on line (choose the "view seat map" option) and select your seat even if you can't print a boarding pass. They can duplicate your boarding pass at the ticket counter but if you don't select a seat at all you might end up in a middle if you wait until you get to the airport. The other option is to buy a "classic" ticket which allows you to pre-reserve your seat at any time.