Hi…I want to create a little fps shooter…My question is: I've load a gun (.obj) and placed it in the bottom right corner, like in many shooter games. But if I walk, the gun should stay in the corner, but it stays at his origin. How I can "fix" the gun, so that it is still at the corner of the cam?
Look up CameraNode, basically you'll want to attach the weapon to the camera and move around via manipulating the camera.
thx… 
I have another question: For the enviroment, (ex: a room), what should I use? (a skybox, terrain…)
Well if you want a room, just create it in a modeling program and import it into JME. Take a look at the TestDotScene.java for loading a OgreXML scene.
Does the attach to the cameraNode works if I use MouseLookHandler? Because I use it, and the weapon stays at his origiin. :?