Edward: It is my experience that a keen understanding one's target market is key. Any ideas for a small business should involve services or goods that you yourself know the most about. You will fail the fastest in products and services that you personally don't use but THINK you know about who uses them and what they want.
When you're 18 you'll qualify to sign contracts and start a business. That
leaves you 5 years to think of an idea.
Work as a consultant to help others become entrepreneurs. Learn about SBA loans and resources, then construct presentations based on that knowledge.
Consulting work can be extremely lucrative. In the course of working with others build their businesses you may find a project that you want to be 100% involved.
If you're that mature-you can start it on your own-or help from parents or someone.
An idea is just a beginning, the key to success is execution and persistence.
If you can invent some product to sell, you can start from there.