Do you have an answer to that question?  You should.  You know why?  Because you can.  And not only can you change the world, you are actually the best person for the job.
I know that might sound absurd, but bear with me for a minute.  You are the best person for the job of changing the world because you are the keeper of the idea/vision/dream/goal inside of you.
I am confident that all of us have inside of us some sort of deep-seeded passion to change our world.  Even if you don't know what yours is, I believe it's in there.  It may be lurking in the depths of your consciousness waiting for the right moment to spring forward from the cave it's hiding in and ruin the perfect world you've built.  More likely, it's waiting patiently for you to discover it.
Now from the abstract to the concrete.  You have a chance to change the world today.  TODAY!  No, you won't make a billion dollars and solve the famine crisis in Africa by 7pm this evening.  But you can start the ball in motion that radically transforms the world.  You see, it starts with you doing something.  Not in the Butterfly Effect, weird sort of way.  I think that by beginning the search for your vision to change the world this very day, you can get on a track that ends up in something far beyond what you thought you were capable of.  But what is the vision you are searching for?  (back to the abstract...)
The author Bill Hybels describes your vision as a "Holy Discontent" in his book by the same title.  That thing that is in your soul that you just cannot allow to continue.  The thing that he uses the old Popeye cartoon to paint the picture- when Popeye got to the point that he HAD to act he would say, "I've had all I can stand, and I can't stands no more."
Your vision is what is going to drive you to change the world.  It's the thing that won't let you sleep at night because you are so excited about the chance tomorrow brings to make it come true.  That sense of "what am I here for?" is answered in your vision.  The change you want to scream, cry, and beg for is waiting to be released by the actions wrapped up in your vision.
The tricky thing about your vision is that it's yours.  Nobody else is responsible for it.  Nobody else has it.  It's just you.  In his book "Visioneering", Andy Stanley says
"Initially, your vision will exceed your competency."
That's not just true for some people.  That's true for all people.  So don't use competency as an excuse.  You can do it.  You have to do it.  You are the only one who can make your vision reality.
Find it, plan it, act on it.  That's how you use your vision to change the world.
Whether your vision is global in scope or the desire to save one single child from a life of pain, you are responsible for finding and implementing the vision.  When you find yourself holding your new baby in Uganda or see the tears roll down the elderly neighbor to whom you just delivered flowers on the anniversary of her late husband's death, you will know that you are on your way to changing the world.
I am no expert on the subject, but I believe that you and you alone have the power to make a huge impact on your world.  You really are the best person for the job of changing the world.  Maybe not in a grand, sweeping way, but in a way that nobody else can.  But you've gotta do SOMETHING.  That's where you start...
I will leave you with a closing quote from Francis of Assisi:
"Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."