SHORT ANSWER: *You need to have a "critical mass" of your supporters (depends on many factors: poverty, oppression, injustice) and then it is like snowball and it's 50% chance you succeed, at least in the short run.*
Creating a revolution is a tough thing if you don't have the media, war machinery and money. Many books were written on that topic.
And where would the revolution start first?
Probably where you wouldn't expect it at all.
I would say something like the USA. E.g. Karl Marx thought that the revolution should begin in the UK or France.
Guess what... he was wrong and it started in "Russia" ;)
But back to why I would say the US is my tip for the next "revolution".
People in the USA are accustomed to a quite big living standard when it comes to the World.
And now they are loosing it for various reasons.
You got the privacy issues including guns, monitoring internet, drones and these things are increasing intensively everyday.
People in the EU are accustomed to such government policy (even we have more demonstrations etc.) but for the US this is something "new".
So, the question is: Could they adapt to lower living standard or not? Probably yes, but the revolution is the other option.
Btw. Most of the revolutions were orchestrated in the past by influential groups and entities. Only a few were "from the bottom". So, if you are thinking about becoming an author of an revolution, your chances are very very very low (but not zero ;), of corse). Instead I would suggest to make some "evolutionary" kind of revolution by changing things you could change (influence other people by your thoughts, things you do, create new things, show what is done wrong and offer alternative etc.). But remember, thoughts are the most dangerous weapon, so be careful).