How to pick your Platform

Unless you are already affiliated with a national company and work for them then a charity is something you assist with your volunteerism. 

I volunteer for United Way, Big Brother/Big Sister

I volunteer for Susan G Koman, and the MS Foundation for ALS

But my platform was "Be a Mentor" worth with Big Brother/Big Sisters

I want to be a mentor for young people but I worked with several organization to do so but the main "Charity" I worked with was Big Brothers/Big Sisters.

A platform can be an organization that you started or a cause. Defeating childhood obesity, domestic violence, or PCOS. But in your words such as. Walking out of childhood obesity - promoting children to be active. 

Think of the charity is the box you stand on when you proclaim your platform. Be passionate about it and make sure you understand that it needs to be something you can promote nationally. Not just something in your neighborhood.