Hello! 

I don't know how to work Minecraft (in general or on this site), so could some tell me how to use multiplayer? 

Thanks in advance ! <3

Using webclient: sign into classicube.net then go to the server list, and click a server name to join it. 

Using stand-alone client (recommended): Download the game, move it to a folder where you can find it later, then run it. Sign into the launcher, then click join on the server you want.

You can:

1. Go to http://classicube.net/server/list and select the server you want to play on.
2. Download the game, sing in and select  the server.

You must have a ClassiCube account and make sure you logged in.