If possible, pick a flat one! If you select one with hills, then make sure you are running hills during your training runs.
Some have lots of people and some have far fewer people. There are advantages and disadvantages to both. If you select a marathon with a lot of people (perhaps more than 10,000 people), then you can expect some traffic in the beginning, but you will have a lot more fan support and people to run with during the later stages of the marathon. On the other hand, if you run a smaller marathon (perhaps less than 500 people), then you may enjoy little, if any, traffic early on, but find yourself at mile-22 and be all alone with not a runner in site. We all have different preferences. Select the one that is right for you.