Don't donate to any organized cause. Even the best run, most efficient ones still have part of your dollar go to administrative or marketing costs.
As you move through life, you will meet plenty of people that need help. Give that pan handler on the side of the road a hamburger. Help your single-working-mother neighbor by paying for a baby sitter so she can have a night out. Buy groceries for the person in line at the store behind you that is using food stamps.
Or, donate your time. Join Habitat for Humanity and build a house for someone.
While all these options take more time/effort than just entering your credit card into a website, those donations of money/time will be completely dedicated to the person in need.