Your why is your purpose.
What makes you angry?
With so much hate filling the news it shouldn't be a problem to figure out what makes you mad. Famine, sickness, homelessness, waste, child abuse, dirty water, the education system, bullying, suicide, the list goes on. I don't know anyone who is happy about these issues. Who hears a news report of a child getting shot or overdosing on drugs and smiles about it? If you don't know your purpose pick an issue that makes you angry and change it!
Gandhi is credited with saying, "be the change you wish to see in the world." This means not only picking an issue and do something, anything, to make a difference but it also means living out that difference in your own life.
Our why's are not just words spoken they have to be actions shown. We can't say we want the world to have peace and then start fights and arguments. Changing the world also means changing yourself!
What brings you joy?
The next question to ask is what makes you happy. What are the things that bring you joy? Do you find joy in art, writing, planning, helping people, the outdoors, kids, old people. If something brings you joy use it to change the world. There is a quote that says, "if you love what you do you will never work a day in your life." Find what you love and do it.
The world needs more joy and more passion. Sometimes we think our ideas are crazy or won't work. A popular YouTuber named Casey Neistat had a campaign with Samsung that said, "do what you can't." You can watch his videos here.
It may seem impossible but if you believe in your idea then try it. If you put your energy and passion into something and it fails at least you won't have any regrets of not trying. Instead you will know how to make it better for the next try. Do things because you love to do them! It's not about money or fame. Just do something.
What are the things you wish would change? What are the things you love?
How do you use that to change the world?