At the same time you are figuring out where you want to sit you want to be thinking about where you don't want to sit.
To get the legroom you want or need you'll see where the premium seats are. You won't see their pricing but based on where they are on the plane you'll have some idea if it's something you're interested in. If legroom is the big deciding factor for you, avoid seats in front of the exit rows. That extra legroom in the window exit aisles has to come from somewhere.
Avoid seats near lavatories, while this is self explanatory, consider too the lines that form near them making it harder to get in and out of your seat.
Ever been seated in the last row aka seat hell? In most cases the seats don't recline and on a long flight this can be long term discomfort. It's going to be next to the galley or the lavatories. Galley lights are never turned off, it's where flight attendants congregate and see above for lavatory issues. Another thing, if you're even a little bit claustrophobic, a last row seat can bring on the terrors. You're one of the last people off the plane and the main aisle is clogged with people until it's your turn. So if you are claustrophobic grab that seat near the front exit if at all possible. And if that's not possible, make your case to the gate agent. If the seat is available they'll give it to you.
Bulkhead seats are always in demand but just remember that EVERYTHING has to go in the overhead because you don't have a seat in front of you. And this always means a briefcase and often means a purse, diaper bag etc.
And just a word about middle seats...I think they get a bad rap. I've flown Southwest many times and always seem to be one of the last to board because I always forget to check in 24 hours in advance. And if you don't check in within 24 hours to the minute, you're in Boarding Section C #64. So when I board I can march to the back of the plane or take a middle seat in Row 3. Now, I always disappoint the people in that row who thought they were going to have an empty middle seat to put their things in but I've had some of the best conversations on those flights. In fact in 2008 I sat next to a commodities broker who told me to buy gold just before the gold market went crazy. If only I'd listened! I digress. One other perk, I was off the plane 2 minutes after the door opened and on my way home before some made their way to baggage claim.
Remember that most airlines keep up to 10% of the seats on a flight unassigned until the day of travel so they can provide seating for someone in a wheelchair or someone with small children. So if you book your flight after the airline has closed seat assignments that doesn't necessarily mean you'll get the worst seat on the plane but my advice to you is get to the airport early and ask the ticket agent what is available.
Happy Trails to you.