Second, everyone can be a "genius" as long as they put a lot of work into anything. It doesn't take much to become better than the average person - you just have to be slightly better.
Once you start working on something for a long time and thinking about it more than 50% (arbitrary, but about right) of all your waking time, your brain will dedicate a big part of its new neurons and synapses towards that, leading to new thought that would've otherwise never occurred.
Genius also directly relates to discipline - if you don't have the self-control (whether through willpower or some sort of OCD-like disorder) to learn and create something, anything, you'll be just average.
I don't mean Leonardo Da Vinci and others who created new processes for painting - I mean specifically the squares, buckets of paint on canvas and Picasso-like LSD-induced paintings, which are just works of art, not genius. Any decent artist with Photoshop can do better nowadays.