McGill physics machines mostly run Debian Linux with KDE window managers. So sitting in front of an open terminal, the first thing you see is the LOGIN window. Interface is intuitive. Just type in your user-name and password and you are logged in.
Everyone entering McGill is given a DAS account. But to access physics computers, you must have a physics account. If you don't have a physics account, ask your supervisor to get you one.
If you don't have a desktop set up on your desk yet, there is a common computer room on the second floor, room 230. To get in, press 1,3,5, in that order. There are also few Windows machines in that room as well as a couple of scanners.