You don't download Half-Life 1 in the way you're probably thinking. It's a commercial game, which you must purchase.
Your local electronic/video game store should have it, you can order it off the Internet, or you can order it directly from Steam, (the Valve Premier Pack) the program you'll be using to play online. Just don't buy a used copy, as you'll probably get swindled out of a CD key, which is the single most important thing you need if you're going to play online.
do u actually know where i can download it though because i can't buy it yet cause it's bank holiday and i despretly need it today if u don't know where to get it does esf work with the half life demo ?
To answer your question, ESF can only be played on Half-Life 1.
For future reference, don't ask for warez. Downloading a copy of Half-Life is - incase you didn't know - illegal, the forum will have no part in your attempts at thievery. Do it again and you will be warned.