For Farcry: Sandbox is included. And manuals for it. Download other help guides like Aeon&Swoops guide, Ben's Small MP Bible, etc. [Link for those on my site under 'Links']
Aeon&Swoop Tutorial:
http://www.crymod.com/uploads/tutori...g_FAQ_2.19.pdf
Ben's Small MP Bible [For all games, not just Farcry]:
http://benb-design.net/article02/benssmallbible_1.htm
Photoshop Customization tips:
http://www.grafx-design.com/psp_tut.html
Adobe Photoshop .DDS Plugin[For skin/loadscreen editing]:
http://farcry.filefront.com/file/Ado...S_Plugin;37136
If you want to customize more or import made models from like Maya or 3DSMAX, you'll need the SDK. You can get that and the other tutorials from Filefront.
For Halflife 2: Use Steam to install the Hammer program and the SDK. Get help from Wiki at HL2World.com under the 'Tutorials' Section.
For Unreal Tournament 2004: SDK and map-creation software comes with the retail game. UnrealED. Get help from Planetunreal.com/forums or Planetquake.com/polycount. Here's a link for UT2004 modding and much more:
http://udn.epicgames.com/Main/WebHome
For Quake 3: A little old, but still fun. Get help from Planetquake.com/forums and Planetquake.com/polycount. Haven't mapped in Q3 for a long time, but it's pretty easy. Much easier than Farcry or HL2. But I think lighting is more important in Q3.
Most importantly, I think, is experimentation. If you don't know what a button/option does, try it! The most important thing in mapping I think is A SOLID PLAN OF WHAT YOU WANT TO DO. For this, read Ben's Small MP Bible. It goes into great detail. I am currently done with a TDM Farcry map that illustrates this perfectly. Screens of that later. Good luck mapping/modding/customization/etc.!