Well now Counter Strike 1.6 only requires a copy of Half Life. With the mention of HL2, i assume you are speaking of CSS, or Counter Strike Source, that runs on the new engine. Now for that, first you obviously need a steam acount, which can easily be obtained. Now you have a few choices:
You could purchase something like this
http://www.play.com/play247.asp?pa=sr&page=title&r=PC&title=734644
Or you could buy CSS, or a Half Life 2 pack through steam.
The CSS pack is quite obviously, CSS.
The HL2 pack comes in three flavours:
Bronze: Half Life 2, CSS, Lost Coast (highly demanding HDR tech demo)
Silver: HL2, CSS, DOD Source (day of defeat in the new engine), Lost Coast.
Gold: Everything previously mentioned, plus lots of unnecissary memrobilia.
Now even though your just after CSS, i strongly reccomend the bronze pack, as HL2 is a decent game in itself, and allows for mods such as the immensely fun and versatile "Gary's Mod".
Hope that helps!
