If you aren't talking about too much money, try
If you can't convince a family member to have faith in you, its pretty hard to convince strangers.
If your family doesn't have resources to help, look in your local newspaper for venture capital classified ads. Make sure you have a business plan written up before you present your idea to strangers.
If you need help writing a business plan, be sure to check your local library for books on business plans, and follow one of the examples in the book.
Also, be sure to look presentable when you are pitching your idea. Just because you are 18 doesn't mean you should look inexperienced when you meet to discuss financing.