Enjoy Half-Life? Then you'll enjoy making your own custom levels. Think Half-Life sucks? Well next time its gonna be your fault. Make your own levels; prove you can do better than the guys at Sierra. First, You'll need to get Worldcraft 3.4, also known as the Valve Hammer editor now. But first, you gotta know how to use the console. Click here to learn how to use the console. Once you're familiar with it, and can input some commands, download Worldcraft 3.4.. There are also some tutorials you can use get you started there. I can't provide tutorials or even information on how to actually get started on level editing; I'm too busy making my own ;^)
Anyways, good luck and have fun with level editing! If you didn't come here to leanr how to make custom maps, I don't know why you're on this page anyways.