The system tray is located at the very bottom of your desktop, in the taskbar (which is the long bar that runs across your desktop). If you look along the taskbar, you'll see some icons in the quick launch area, then you have some open space for any programs you might have open. On the very far right, you'll see where the system tray sits.
The system tray contains the clock icon, the volume icon and a variety of others, depending on what programs you have installed on your computer. Usually, your antivirus program icon will be there, it's where your printer icon shows up, your modem icon is there, the safely remove hardware icon sits there and so on. If you need to use any of the items in your system tray, all you have to do is double click with a mouse, or press enter on the icons if using a screen reader and the programs will open. Right clicking on these icons can often bring up several options as well.
If you use a screen reader, hold down the Windows or Start Menu key and press the letter m. This will take you to the desktop. Pressing tab will cycle you through the Desktop, Start Button, the Taskbar and the System Tray.