If you have windows on your keyboard is the windows symbol. Its on the lower left corner next to the Alt key. Hit that button then use your arrow keys to get to shut down.
Or try pushing Alt+F4 to close programs one by one until you close down windows.
Modern computers usually also shut down when you press the power button. Just press it once, but don't hold it down and the computer should start closing programs and shut itself down. Though some do go into standby instead. You can alter this behaviour by going into the control panel (also accessible in the start menu, under "Settings" if not visible.) and opening "Power Options". It is under the "Advanced" tab at the bottom.