Don't go to New Game.
Find Argonath server in the SAMP window and then copy the "IP Address" click the button at the top "Add to Favorites" and paste the "IP Address".
Then Argonath server is in your "Favorites". Then connect to the server, let GTA:SA load up and it will load up to the server.
Then use "/register (password here)" and then "/login (password here)" and you can play

If you can't get onto the server, you need to downgrade your version of GTA: SA with the downgrader patch. I can't remember where you can get it but I'm sure you can google it, or someone else will post a link here.