Get steam at:
http://www.steampowered.org (im pretty sure)
Once that is, run steam. A menu should pop up. Click Play games, then right click Half Life, and install. If it already is installed, then click play game. Play it once, then get the 1.2.3 installation of ESF at
http://www.esforces.com
Install that into: steam\steamapps\
[email protected]\half-life. Install, then open Steam (if the play games window while you installed and afyter, then open and close). Then under '3rd party games' there should be a game labeled 'Earth's special Forces'. Double click and a loading thing should come up 'Loading Half-life'. This is normal, just wait and let it load and play.
If you honestly cannot understand this, PM me or something. I'll hlep you step my step.