Summarize this article:

If you ordered your ticket online through any website, you'll find this code in the email receipt/confirmation message. If you have a physical ticket or boarding pass, this code can be found on the front.  Depending on where you purchased the ticket, the code may be referred to as a "Record locator" or "Confirmation code." Even if you initially booked your flight through a travel company (such as Expedia or Orbitz), you can still find your flight information on the airline's website. You can access this website on a computer, phone, or tablet. It's between the "Book flights" and "Flight status" tabs. If you know your flight number and just want to check its estimated arrival or departure time, click the Flight status tab instead, enter the flight's details, and then click Find flight. Each passenger should be checked in separately. Enter the name exactly as it was entered at booking time. This displays information about the reservation, including the flight number, departure/arrival times, seating assignments, and upgrades. You'll be prompted to sign in to your American Airlines account if you aren't already signed in. If you don't have an account, you'll be asked to create one. Depending on when and how you bought your ticket, you may be able to modify your seating, meal, and timing preferences. If you need to change something and don't see the Change trip option, contact American Airlines reservations for assistance. Agents are available 24/7. You can find the phone number for your region by clicking this link. American Airlines allows you to check in for your flight online up to 24 hours in advance. Checking in online allows you to skip the curbside, desk, or self-service kiosk check-in options at the airport.

Summary:
Locate the 6-digit code record locator. Navigate to https://www.aa.com. Click the Your trips / Check in tab. Enter the passenger name and 6-digit code. Click the blue Find your trip button. Click Change trip to modify your reservation. Follow the on-screen instructions to check in for your flight (optional).