The most important thing you can do as an adult is to decide exactly who you want to be. It might sound cliche, or overly simple, but it is so important to determining your path.
If you do not have that written out and set in terms that are black and white, then your direction will be far less clear. When that happens, you begin to leave your path up to fate rather than your own planning.
It's also important because it can determine who you allow to come into your life and who you don't. As children, we become trained to accept relationships of convenience. Who we hang out with and who we spend our time with is largely up to our family and our geographic surroundings.
As an adult, you get to choose who you spend your time with, but if you do not choose wisely, it can pose limitations on yourself and what you're going to do with your life.
If, however, you've decided who you want to be and what your values are, it gives you a clearer definition of what you'll tolerate from yourself and from other people.