1. Go to http://satracker.sebszhost.com/newaccount.php and create an account. (You won't be bugged with emails)
2. Start SATracker.exe, enter your username and password and click GO.
3. Now start GTA: San Andreas. Start a new game or load a save game, it doesn't matter.
4. Once you are in the game, alt+tab out of San Andreas. Check what SA Tracker has to say:

If everything went well you should see:

"Game Status: San Andreas is running..." - If you don't see that, make sure started gta_sa.exe.
"Overall Status: Tracking..." - If you don't see that, make sure your internet connection is working, make sure you started San Andreas (see above) and make sure you are inside a game (able to move around)
"Logged in" - If you see "Authentication failure", it means that your password is incorrect, or the username doesn't exist.

Any questions or suggestions can go to me@sebsz.com.

Enjoy