I would definitely start out with some smaller races - 5K, 10K and then work your way up from there. If you haven't done a race before the distance could surprise (or not) you.
What's your fitness background like? If you already workout regularly, you might be able to transition easily between distances. A couple of long time runners I know have told me that they spent a few years on smaller distances and gradually transitioned to a marathon. For me, (in my old age

) that seems to be the best plan. That said, I know a lot of people who've jumped right into a marathon without doing any other races.
Whatever you decide, my adivce is to make sure you go to your local running store and get fitted for a proper pair of shoes. Good running socks also make a big difference too but they aren't cheap. Also make sure you integrate some type of cross training into your running routine and don't forget to do a core workout. From experience, your knees, back and hips will thank-you
Good luck! And keep us posted on your progress!