If you have ideas for a businesses, research those ideas online. It is a great idea to have a place to jot down the ideas and build out the plans as you research and learn more and more. If you identify one that has potential (think 5 P's - Product, Price, Promotion, Placement & People), you can put together a business plan. That may seem a little daunting, but it is really easy to do and can help you think through everything you will need to cover.
There are tons of resources online for this from the Small Business Administration as well as others.
You might also consider looking into a few groups that are focused on entrepreneurship - Junior Achievement is one.
There are a large number of people who started young and created businesses, but also many young people creating non-profits built on ideas that are very important to them (the environment, access to books, etc.)
Have fun and I hope you are successful in your venture.
Once you have a business idea you move to planning, ask yourself "How can I make this happen?"
It's ok if you can't do it just by yourself, part of your planning should be finding those who can help you with it.
Throughout the whole time you find a business idea, plan, and make it a reality, age should not be limiting you from achieving your dreams.