Really basic Linux question

I have a Linux practice server, to learn on, with the idea of moving to Linux in the future. I tried to access or do something awhile back (I don’t remember what) and got a not authorized error. So I looked into the groups and saw adm, which I took to be administrators. I put my user name in that group, thinking that should give me access to everything, like in Windows. When I rebooted the server I couldn’t log in with any user name at all.

Anybody know what I did wrong? I figure it’s a really basic mistake…