Also, you need a valid copy of Half-Life for playing ESF with Steam. That can be obtained in 3 ways:
1. Buying it in a store, which is usually only found in packs, like Generation and Anthology packs, and they're usually more expensive than the game itself.
2. Buying it online with Steam, costs $10, the cheapest you'll ever find it.
3. Get it from someone as a gift.
Also, I suggest you download
Steam with Half-Life cache. It already has Half-Life inside it but you need the CD key to extract it nonetheless. The advantage of this is that it's 200 MB, unlike Half-Life which is 500 MB if you download it through Steam. Also, if you wish to uninstall it, you can install it again if you get the wish to play ESF again without having to download Half-Life again. Although, you will still need to download updates for the game, so the download will start around 94%. Do not ask for help if you are using WON and/or CS No-Steam, at least not here, because this forum supports only Steam, simply because other methods are hackable, and warez aren't allowed here. That would be it. And read all these stickies before asking more questions, you might just get your answer:
How to install ESF
Frequently Asked Questions
Newbies Guide to the ESF Forums
How to ask for help
Acceptable Use Policy (AUP)