There are a million ways in which we define ourselves and those around us. Race, gender, age, location, hair color, hobbies, shared and separate experiences... the more you look, the more ways you can find to define yourself as part of a group, or separate from a group.
This exercise isn't about prejudice or bigotry.
It can be, but your story is yours to tell. No exercise will define it for you.
What we're looking for is a moment, or series of moments where you experienced the feeling of otherness, of being separate from a person or a group. It could be forced upon you, a realization you have, something that definitively set you apart. Write a poem where you explore what it was like, what you experienced.
Check out “You are in the dark, in the car...” by Claudia Rankine.