First real post!
Getting the wrong seat on a plane can be a nightmare, but happily it’s something you can prevent most of the time with a little bit of research.
The first stop when picking a seat should always be seatguru.com. Match the type of plane you’re on with what is available for your airline and you’ll be able to check out exactly you’ll be on. The site gives you a great overview of the best and the worst seats for your plane including warnings about window seats without windows and tips about which seats don’t recline at all, all of which sometimes don’t show up on the seat maps the airline provides. If there's one take away from the site though, it's that of all the seats, avoid the last row. These virtually always have no recline.
On of the biggest mistakes that people make is to assume that exit rows are automatically better than other seats. While it is true that they offer more leg room, they often come with the downside of either being narrower (when there's no seat in front of it, just a bulk head) or completely unable to recline (generally when there are two exit rows over the wing, the first row will not recline)! Double check to make sure that the seats recline, or you may end up sitting straight up the entire flight.
Whether you choose an aisle or window seat is up to personal preference: Window seats have the advantage of easier sleeping and views, but getting out to walk around is understandably difficult. You also avoid the risk of having an elbow knocked by the food cart or a foot trampled by people walking in the aisle. On the other hand, the aisle is better for people who like to get up a lot or need to get up a lot to stretch their legs. On longer flights, this can be a real advantage if you are unable to fall asleep.
Most planes are 3/3 configurations, meaning there are three seats on both sides, with two completely unwanted middle seats on both sides. For these the above rules apply, but on longer distance planes, you’ll also find 2/4/2 configurations. The two seats on the side are ideal for couples and better for the solo traveler as well.