How do you do concept art?

I'm better at drawing pictures I see in front of me. I'm not really good at drawing things from my head. I always see concept art from games and such and they look good. How can I start making characters and creatures up in my head and putting them down on paper?How do you do concept art?
just come up with a motive and idea and then create the character around that. First think of what your character will be doing and then draw it.