As for your issue. What do you define as "how can I play"? How to "play" the game (control wise, etc) or how to play (launch) it once it is installed?
You need to own Half-Life (on steam) -> https://store.steampowered.com/app/70/HalfLife/ if you don't own the game yet. After you've installed it (and installed ESF) re-launch steam so Earth's Special Forces will pop-up in the games list.