It is next to impossible for bitcoin to be a single world currency.
This emerged in the year 2011 i guess and now it has been only 6 years.
Also it is just a crypto currency / online currency , and there are more important currencies.
Like USD  etc. But as the world is turning digital , bitcoin stands a chance to be a part of the currency.
So although it might play an important role in digital world , it will not be a single currency.

I being a newbie use faucets to earn. Now many people wont know what faucets are.
They are basically apps , by which you can earn some satoshi's by just following the instructions.
According to my information , once anyone becomes a junior member , they can participate in campaigns.
Campaigns are basically the best way of earning bitcoin . They pay you for posting and keeping their signature.
Also my friends told me about an official site freebitco.in
But you earn very little from that.
Every hour you get very less satoshi's.
Its better to use faucets.
When you become very much knowlegeble about bitcoin , you can even gamble.