You don't need to log on to the Macs, everyone uses the 'Student' user name.
If the computer isn't already logged on, click on the 'Student' user.
At the bottom of the screen there's a bar with icons for all the applications you can use. This is called the 'dock'. The dock also contains the trash can - drag a file into the trash if you want to delete it.
On the right hand side of the screen there are several icons. 'Scratch' is the folder where you save your work while using a computer, and 'Connect to Media/Network Drives' allows you to access your network space to store files more permanently.
Note: please do not work on files from the Media or Network drives or from a USB Pen copy them to Scratch. This will help prevent any latency/slow running caused by network/bus speed limitations. This is particularly important for working with audio or video.