During your teenage years, you may not feel that you have much direction and you aren't really sure where you life is headed. You are sort of standing at the crossroads of life thinking "now do I turn left or right?" Life is pretty much what you make it and you can make your prospects better if you work hard towards them while you are young. I began working towards my future when I was 15 turning 16. My best friend got a job at an Internet provider that paid above minimum wage to start. I didn't see him for 2 weeks straight and finally he took a Sunday off. I asked him if he could get me a job there, and he did get me in. That's when I started focusing on Computers as a career.
When you start out deciding where you want to be in about 10 years, plan realistically. Realize that you aren't going to become an astronaut overnight and that it will take a lot of time and qualifications to reach that kind of level. You must plan and start to build up experience in the field you want to go into. Volunteering is that way to see if you really want to go into that field, gain experience, and give back to your community.
Say you want to become a doctor when you are older. The medical profession has always interested you and that is the career path that you have wanted to follow since you were a little toddler wrapping bandages around your cuts, bruises, and other's wounds. Doctors need a College Degree/University Degree and good qualifications if they are going to become successful. You could start out positively on this path by volunteering at your local hospital for a few hours a week, learning how to care for patients with all sorts of different problems and most importantly gaining the experience that you will need in later life.
Maybe the thought of diagnosing the sick isn't the choice for you and you'd rather become a youth leader or a teacher. This job requires being able to work with children/young adults and connect with them on a level of understanding so they can learn. Start out on this path by volunteering at your school in various clubs which have an impact on the way the school is run. Some good examples of these are: the school newspaper, the graduation committee, or the student council. You will be able to gain that vital life experience needed so when you present your resume to a prospective employer, you will be able to list all of the different places you have helped out in.
Another example is if you want to work in a library when you are older or as a researcher then start by visiting your school library and see if they need anyone to carry out menial tasks such as shelving, labeling books, organizing stock and sending out invoices.
Building up experience while you still have the time on your hands (in your teens and early 20's) is crucial and really can give you a good starting block for your life. Just because you aren't getting paid for these jobs, doesn't mean you aren't getting something out of them. :)
I'll leave you with the quote from the Founder of HelpingTeens.org's predecessor site Teenhelp.org, Branden Thomas:
"Volunteers don't necessarily have the time; they just have the heart."