Jon,
Usually shops will let you demo seats. I suggest stopping by your local shop and asking for advice. I'm sure they will go out of their way to help you, knowing that you will likely become a long-term customer at your age. I grew up with a bike shop across the street from my house. They use to do all the minor fixes for free and I used to test ride all of their bikes. I spent a small fortune there and they loved me because of that. Getting started at your age is great and I'm happy that you are a member of our online community. If you have any other questions relating to cycling and fitness, please go ahead and post another discussion question!