In the Windows operating system, click on the "Start Menu" in the bottom left hand corner of the screen and a small menu will pop up in that corner of the screen.
Depending on the version of Windows you are using (XP, Vista, Windows 7, etc.) this menu will appear differently, but in general there is another button towards the bottom-right which either says "Restart" or has a button which expands into another small menu containing the "Restart" option.
If you can not find the "Restart" option, you can accomplish the same thing by choosing the "Shut Down" option and then waiting until the computer is completely shut down (the screen will be black and the actual computer will become quiet ... the fans inside of it will turn off). The press the power button (it looks like a circle with a line cutting into the top of it - here are some examples).
Last Resort Edit
If the tips above aren't working out, a last resort is always to find the power button (examples of what it may look like... it will almost certainly have that symbol on it) and hold the power button down for 5 seconds until the computer shuts off. Wait a few seconds and then press the power button again briefly until you see/hear the computer coming back on.
It is not recommended to do this all the time, but it should suffice for occasional use (like junk food).