How do I shut off the power, so I can install a USB enabled electricity socket?
Yes flipping the switch will turn the power off. You will still want to double check at the outlet with a voltage tester.

The objects to the left are fuses and are color coded to the amperage which you can also see embossed on the front (red is 30 amps; blue is 15 amps and I can't read the white one). If one blows then you will probably want to replace the entire panel for one with resetable fuses as I don't think you'll be able to find replacements easily anymore. It's also possible that a previous owner added a bypass wire at the back of a blown fuse. This is not something you want to keep in place. 

Remember that the power to everything will be cut including lights and your fridge; freezer and computer. If anything can't handle a sudden power outage well then shut it down before cutting the power.

If you have appliances with an induction motor (like a pump or a blower) then the current may spike when you turn the power back on and blow a fuse. If you have several then disconnect them and after you turn the power back on reconnect them one by one.