You can check on www.seatguru.com to find out, but generally "A" seats are window seats. On almost all aircraft, window seats are "A" on one side, but every airline numbers their seats a little differently going across the plane.
For example, American and Delta both fly MD-80's which are 5 seats across.
American's seats are like this:
[window] A B [aisle] D E F [window]
Delta's are like this:
[window] A B C [aisle] D E [window]
TWA's used to be like this:
[window] A C [aisle] D E F [window]
Thanks! I'd heard of seatguru but couldn't remember the name to check. An aisle seat would have been fine, but it's nice to know I'll have a window seat - love to look at the terrain and the cloud formations.
ETA: I just checked out seatguru - wow! At first I couldn't find out the aircraft type from Delta, so I played around and looked at their entire fleet (too much time on my hands?). Then I saw seatguru's tip for figuring out the aircraft type and found my actual seats - and I'm glad I did: for the second leg of my flight I'll be right behind the emergency exit that has the slide, and found that that area can get very cold during the flight. I hate being cold, so I'm glad I know ahead of time to request a blanket.