nice guide blokker, but if i may, im going to add some more.
If you have more than 1 server running, the "screen -r" command will bring up a list of the running processes like this:
If you want to view a specific server, you will need to do "screen -r [pid]", so in my picture for example, "screen -r 19982" would bring up the first server:
But what happens if some cracknut comes onto your server with an exploit panel and crashes it? No more server for anyone until you get it started again? Nope. If you're smart, you would be running a restart script. Mine checks every 10 seconds to see if the server is running, and if its not then it will automatically start it back up again. This basically makes the Shutdown button in MTA:mA act like a restart button.
Also one last command you may find useful is "ps x". This shows you all the running applications on the server: