OK Sir...
Dae is right, get a feel for things by looking at PlanetDeusEx. There are some great fan-made missions there such as Counterfeit, Fatal Weapon and Hotel Carone, which gives you some ideas not in the original game.
I would refer you to
http://www.dxms.co.uk It's a community of Deus Ex mappers. There also you will find very useful tutorials...
Make sure you have SDK final version and Steve Tack's DX Lab. Both are fairly small downloads and Steve Tack, although written for SP, covers almost all of the basics. If you can't access via PDX, try dxediting.com
Some basics not covered by this route are:
1. Align to grid - before you add or subtract a "brush", make sure you right clicked on a corner of the brush, moving the little red cross from the centre, to the corner of the brush. Don't forget to do this for your "initial subtraction"
2. Don't forget to compile ALL scripts.
3. Don't forget to include a "Deus Ex Level Info", found in Classes > Info in your SDK. Don't forget to set that Deus Ex Level Info to "Multiplayer Map = True"
4. Save all the time, especially if you change any kind of lighting
5. The SDK is not the best software in the world, work carefully and methodically, don't invite trouble from the SDK. You'll understand what I mean once you start using it.
6. Make sure your ladders do not touch anything at all.
7. Make sure you make movers in an open space, not touching anything, before it is moved to the desired location.
8. Learn advanced texturing first. Learn to select a brush that is textured, right click on it, select "Properties" and you will see a whole host of features which enable you to resize, flip, scale, pan textures and set properties for them such as transparent, backdrop, mirrored etc.
If you are stuck come and see us at
http://www.dxms.co.uk