Valve has a really nice documentation site for mapping/modding their
engine and games. If you have one of the Half Life 2 games installed
on your machine I highly suggest downloading the Source SDK found
here:
http://developer.valvesoftware.com/wiki/SDK_Installation
and following along the beginning tutorial found here:
http://developer.valvesoftware.com/wiki/Your_First_Map
This is one of the easiest ways to break into 3D game design, as the
only requirements needed to get started (note that being good at this
requires much more) are having played a first person shooter game at
some point and being able to recognize the difference between a
computer that is turned on and one that isn't.
If you've done this and are interested in TF2 mapping, here are some
extra resources to help you along the way.
http://forums.tf2maps.net/showthread.php?t=4674
Installing this into your source sdk folder will give you maps laying
out all the props and gametype node systems for you to copy paste into
your own maps, as well as icons for many of the TF2 game entities.
The source sdk also comes with the official map files for TF2, so you
can see how the Valve team makes maps. Just be warned that these are
full of non-rendered entities and brushes that makes this whole
process seem horribly confusing and difficult. This will just show
you what I meant when suggesting that being good at mapping involves
more than just playing games and using a computer :D