That's amazing. Many successful entrepreneurs have started in their teenage years.

I definitely think a freelance developer should be hired and I interview web designers, so I can act as your CTO and find you the write web developer.

I am not sure you need a co-founder unless you want someone to share your ideas and implement them with you or if you have too many ideas for one person to implement.

The most important piece of advice I can offer is for you to plan.
1. Write a business plan - You can find many templates on the web for business plan.
2. Write a marketing plan which will include viral marketing and social media.
3. Right a finance plan on how you plan to pay investors.

Getting the word out:
1. Find investors once you have your business plan written.
2. Attend angel investor meetings in your area

Getting the Assistance You Need:
1. See if there are other people in your school who would want to join you and help get things done.
2. Hire a lawyer to write you up a partnership deal / agreement / contract.