There are hundreds of ideas that you can utilize to start a business even with a very little budget (you can say without investment) and if your partner is a teen, you should consider below business ideas:
1. Baby Sitting
2. Selling Retouched Goods
3. Selling Old Books Online
4. Writing and Selling eBooks
5. Freelancing (graphics, content writing or coding services)
6. Takeaways (home food delivery)
7. Virtual Assistant
And also blogging, social media management or you can build your own social media networking app, a game, or an app that can help solve a problem on Android or iOS devices.
Everything is doable, just take a start and see what happens.