It's harder than it sounds to pick a topic when you have the option of just about anything in the world to research. So how do you start? With research!
It's not as much of a chicken-and-egg situation as it sounds. (Highly recommend the video over to the right!) Think of something that interests you (a hobby, a passing interest, something that caught your attention in the news). It doesn't have to immediately relate to your assignment!
You start with some background research through Wikipedia or, better...
At this stage, it can be helpful to organize all the things you're learning about in a list or a mind map (see Apps below), which will help you see the connections and where you've been getting lots of details. Your core research (and the argument you're pursuing) should be based on the evidence you've found, rather than picking a stance and seeking specific evidence to support it.
If you try to retroactively cherry-pick sources for your stance, you might be backing yourself into a corner, especially if it's a topic you don't actually know much about. Suddenly, when you try to locate sources, you may find that you just can't find any.
Makes things easier for yourself: go where the research leads you.