Writers have been known to say that ideas are everywhere, and as tempting as it is for me to say the same thing right now, I will resist that and try to offer you a bit more useful explanation on just what goes into the discovery of an idea.
The first thing one needs is focus. Focus on what kind of writing you do. If you enjoy writing about horses, it does not make sense to try to write a story about a car – even if it is a story about a Ford Pinto.
What if you do not have a choice, though? What if you have to write a story about a Ford Pinto? Then you want to find an angle you can write from.
The angle is your approach to the story. The perspective you see it from. Maybe you want to write about the way the car got named after a horse? Or how much horsepower the engine of a Pinto has?
If you must get an idea for something from a certain item, then you want to look at that item from as many different perspectives as you can and find the angle that fascinates you and gets your writing going.
If you are just wanting to write about anything, with no assigned writing subject, then dig deep and find the things that interest you. Failing that, grab the newspaper or surf the internet newspapers for a little while and search for that spark of inspiration that says "write about that!"