How to Decide What to Work On
You should pursue a project if and only if it:
Maximizes your values
Is valuable to the world
Is something you are (preferably particularly) good at
Is something you enjoy
Is something you are willing to dedicate at least 10 years of your life to
How to Discover Your Values
Sit down and write out answers to the following questions:
If you were the only human being in the universe, what would you do?
Who are the greatest humans who have ever lived?
What is it about these people that makes them great?
How to Operate
Create for yourself an explicit value system. I like to say that this is the most important thing you can ever do, because it enables you to make every other decision in your life. Why are you reading this right now? What criteria are you using to determine if each potential action is worthwhile? Having a value system makes answering these questions possible.
Live with a sense of urgency. Life is short, and if you're going to do something significant with your life, you must focus.
Be ambitious -- more ambitious than average people think you should be -- especially if you are particularly capable.
Create exciting, specific, ambitious goals. Choose your projects very carefully, engage in deep strategy, then execute on that plan. (That doesn't mean that you don't update your plans as you learn more information, of course.)
Track progress. Is what you're doing working? No way to know unless you measure. Y Combinator tells their companies and non-profits to measure progress every week.