So, you want to become a Concept artist?
I think that is nice, but, you know anything about concept art?
Have you made some research?
You clearly have a lot work to do in the area, would be nice you see a lot of references and how to draw them.
Don't try to be perfect in the fase, just concentrate in studying the human body per exemple, how it work, etc..
A thing that help is silhouettes, you need to do a lot.
Try to do at least 50 per week, in a lot of different positions, etc.
Don't mind if you have mistakes, thats not important now.
What really matters now is you to loose your hand, don't worry.