A lot more than you think! Let me explain...
Every career, from garbage hauler to president, can be defined in terms of the services offered for others. Garbage haulers take your garbage from your curb to the dumpsite so you don't have to. Factory workers build things for you (such as cars) so you don't have to. The president helps lead the country so you don't have to. Everybody does you a service. Whatever you do for a living, you offer others a service. As we all do services for each other, we make life easier and more comfortable for each other.
Now, there's a vast number of careers out there, and many more can be invented. So you aren't stuck in one career or job. Ask yourself what services you would like to offer others, and then start making the preparations for entering the appropriate career.
At this point, several questions come to mind. Some are practical: "How do I get the money for college?" Practical questions require a lot of research, planning, and perseverance. The solution for practical questions is to read books about resolving the obstacles you face, and then apply some of the ideas you read. Reading is certainly one of the secrets to my own success. (Oops--I gave away a secret!)
Other questions are based on fear or doubt: "What if I don't have the brains or talent to do that career well?" The questions themselves must be questioned: Who said anything about doing it well? Who said I can't increase my intelligence or talent? Who said I can't do a decent job if I love what I do? Am I going to stop myself from providing the service I want to provide? Am I going to let others talk me out of it? Would I rather insist on perfectionism or on doing what I love to do for a living? And, wouldn't others benefit more if I give them a service gladly than if I give a service I don't want to give?
Remember that most people only use 10% or less of their abilities. That means that most of your abilities lie dormant, unused, and invisible. Exercise new muscles to make them grow big and strong.
Write yourself a plan for what services you want to offer, what career you'll use to offer them, and how you will build your skills in that area. Then, do something--anything--to start putting part of your plan into action.
Of course, you won't necessarily become one of the best in your field. For example, you can decide to entertain spectators by being a basketball player, but you won't necessarily become a professional player. You can decide to help find a cure for cancer, but you won't necessarily get the Nobel Peace Prize. That's okay. Being the best and being famous are not important. Providing a good service and enjoying it are.
This applies not only to your career, but your hobbies, personal relationships, and every other area of your life.