you just unzip the zip into your half-life directory, the folders should be created automaticaly. But for some mods, if your runnin HL-Game of the year edition, they wont work.
and how to run the mod:
run the hl executable (.exe) and from menu choose custom game then choose the mod from the list, then choose new game or hazard course (etc.)
that should be it, if that doesn't work i could give you the directory structure of a mod, if that helps.