As you are probably aware the world isn't perfect. There are lots of people in the world who could use a helping hand. Those of us in North America have an excess of wealth and time. This makes us perfect agents of change, all we need to do is choose what to help with and who to help.
For now I'm going to assume you've decided to help. I may write more later trying to convince you, but let's just go with it at the moment.
It's impossible to help everyone and have a huge impact. Thus the goal must be helping the most people AND having the greatest impact. But I would argue that this is not a linear relationship.
That is to say that helping a lot of people but only have a small impact is not equal to helping a few people but having a huge impact. I personally believe it would be better to get one man fishing gear and lessons on catching, cleaning and cooking fish, then to use that money to buy fishes and feed let's say 100 people. Money is a bad metric for this because there are occasions where you can use money more efficiently and have a greater impact, but I think it illustrates my point. I believe that helping one person a lot is better then helping a lot of people only a little.
From a mathematical standpoint this may not make sense, but this is humans we're talking about. People are better able to connect with just a few people and thus will probably put more net help into the project. If you compare that to someone who is helping a mass of people who doesn't connect with them will, odds are they will loose interest more quickly and thus contribute less.
Essentially what I'm trying to encourage people to do is to pick a cause that they can connect with or feel particularity close to and dedicate themselves to it. Don't worry about the fact that there are lots of other causes out there. You will NEVER be able to help everyone. Trying to figure out how to best maximize your efforts is a waste. Most people end up giving up and nobody ends up getting helped. So just starting helping already!