Strange shading on my objects! *picture*

Hi!

I added some lights to my existing game level, but all the objects are all reflective, and the shadows seem wrong. Here is a picture of my scene (in game mode),

With this one I added a directional light because my scene was too dark. As you can see some objects are really dark, while other areas are really bright. When I turn down the intensity it too dark. In addition the walls on the left side and back are all messed up. Do I need to add some shaders to my objects? A duffuse one maybe? I have no idea how to do thoes yet, but I could figure out how. What do you think? Thanks (I hope I'm making myself clear it's tought to explain =P)

-Rov