How does the brain work??

Umm... Dude, answering your question is the main focus of psychology. It hasn't been answered completely.

The using 15 or 10% of the brain claim is a wives' tale. We use the whole brain, just activity levels in each area vary depending what we're doing (homework, working out, waiting in line at a grocery store, etc.). We know how parts of the brain function, like the general functions of the lobes and different cortices, but they're always finding out something new. Look up Wernicke's area, Broca's area, and the thalamus to get an idea of how specific the knowledge can be and how confusing some parts of the brain can be.