Hey Bodhi,
All you realy have to do it navigate to
http://www.steampowered.com and select the download option, should be about 600kb or something small like that, it then downloads the file, then just execute it.
Steam will update automatically and then ask you to signup / login if you are a previous member, you just hit register and then follow the prompts, near the end, it will ask you for your halflife cd key, just pop that in and steam will verify it and make sure its not been used on the steam system before.
If the validation is complete, steam will download all the needed files for you, and if you hit Play Games, counterstrike should be there, then its just a double click, and your into CS Online Gaming.
HTH
Chris,