If you're not checking in luggage, just go directly to gate and check in. Have your paper itinerary, i.d. and Rapid Rewards (if you're a member) ready. Then you'll get a plastic pass that is numbered and also a boarding ticket with your name on it. Numbers 1-30 get boarded first, then numbers 31-60, then 61-90, and then 90 on up (if there are more than 90 passengers). You are not seated in order of pass number, but rather by group number. So you'll want to begin getting in line around 30 min or so prior to departure, or when everyone else begins to line up. As you board, you'll be required to turn in your plastic pass and show your i.d. along with your boarding ticket. That's how they ensure that the right passenger is boarding.
Good luck.