MercyKiller said:
Great a tutorial that would make it easier *jumps for joy 4 the sdk*

but isn't it difficult?? i mean for a guy with no experience?
Depends on your background. If you have 3d modeling experience, texture experience, you should be on your way pretty quick, that's not to say you will still have to learn the do's and don't s of creating levels technically.
For a total unexperienced guy, it's hard probably. You will have to grasp the concepts of 3d as well as gameplay, lighting, flow of maps, and all that in a 'realism' kind of enviroment.
You won't pickup up on Level design in a month or 2 or something, it needs time, experience and patience depending on what kind of quality you want.
If you are really into learning it, nothing should stop you from makiing test levels, small ones... and once you have more experience try your hand on your first small map that you will share with others. But before you get there you will need to learn about optimization and flow, the two main specific aspects of level design if you are looking to make multiplayer maps.