So i smashed my brains trying to make my character move. As i wanted later in the game to add explosions and bullets it wasn't a good idea to mess with the velocity and the forces/impulses didn't work as i expected so something stuck in my mind: Is it a good idea to put at his bottom a wheel(circle) which is invisible to the player that will do the movement by rotation? I will attach this to my main body with a revolute joint but i don't really know how to make the main body and wheel body to don't collide one with each other since funny things can happen. What is your oppinion?
That is exactly what you should. I once wrote a tutorial on it for C#/XNA and Farseer, but you should be able to adopt it easily to Java and Box2D (Farseer is based on Box2D).
Here's the tutorial:
The end result should be something like this: