First, get steam. Second, download Earth Special Forces v1.2.3. And third, read the How to install ESF thread. I think this should help you pretty nicely. You get the Client error due you are either not using Steam, or you are trying to connect to a server that has a different Client Version then you. Downloading v1.2.3 complete install mostly fixes the problem.
That's because you have to install Half-Life first, download Earth Special Forces and Install it. Once you have done that, restart Steam and ESF should be in the list. But make sure you run Half-Life once before you are going to install ESF.