Going back to our problem statement:
Obese people need judgment-free and peer-supportive interventions to promote more healthful eating habits.
Start with paper prototypes.
Later you can experiment with wireframes using Powerpoint.
For High Fidelity Prototypes, feel free to use InVision Studio (it's free) if you're designing a webpage or mobile application.
Remember to test your prototypes with potential users or with your peers.
Discuss your solutions in the discussion forum.