If you want to help people, start a business, not a nonprofit
At some point most, but not all, successful people figure out that no amount of success or money will actually make them happy, and at that significant turning point, people start to look for meaning in different places. There are a lot of places to turn, as the human animal can make literally anything meaningful, but the best of these people turn to helping others.
If you haven't done a ton of helping others up to that point, it can look like the non-profit world is the route to take here, as it seems like common sense that helping people is why nonprofits exist. That's fine, and many great people work in the nonprofit world, but if you're looking to really help people it's the wrong direction.
Charity is a very small pond.
If you add up all the charitable giving in the world, including all the endowments, the foundations, the $18 to Bernie Sanders' campaign, literally all of it, it all adds up to 0.005 of the world's spending, a number which has been relatively steady the past 50 years. A half a percent.
Bracket out for a second the moral statement that level of species-wide charitable investment makes, and let's just focus on the practical implications for someone really looking for the best way to help people. Absent running for public office (which godspeed to those souls who jump in that water but I have both no interest and a checkered past) we basically have two choices: business, responsible for 99.5% of all monetary impact in the world, and charity, responsible for 0.5%. Remember the all-importance of market size, and choose to fish where the fish are.
If you still think a nonprofit is the way to go, consider that since there is not enough money to go around, and since there are over 1.5 million registered worthy causes in the US alone, in order for you to make the impact you want to make with your nonprofit, you have to literally take funding from another nonprofit with another well meaning mission to change the world. People or organizations with money and a conscience tend to organize their giving by first setting a budget, and then choosing where to give or which cause to support from that budget. So if your neighbor wins and gets funding, that actually in reality lowers the chances of you getting the funding you need. Charity is literally a zero-sum game, so there's a tangible human cost to any success you have in that area. Not the case in business, in which there is absolutely such a thing as a win/win/win situation (despite, admittedly, some confusion).
So far as I can tell, the only way to create lasting meaning for human beings is to help other human beings. For those of us who have learned the emptiness of self-serving success and want to create meaning by helping others, start a business, not a nonprofit.