leo-
If you have a copy of Half-Life, you should just be able to double-click the esf installation file, and keep on clicking next.
After that, get Gamespy Arcade (and install it and such). When you do that, open it up, go to the Half-Life section, select all game servers. In the filters box for "Game" type
esforces. It should bring up all of the servers for ESF, and you should be ready to go. If you want to change your name and stuff, you have to go into Half-Life, load a custom game (ESF) and configure it from in there.
This is just one, very brief way to do it. There are other options. This seems like the fastest to me. Maybe it's just the cable though...
Good Luck!