I don't know how to get my Yongnuo flash to work on my Nikon D5100?

I bought a Yongnuo 560 mk2 external flash for my Nikon D5100 and when I put it on the hot shoe and take a picture the flash doesn't go off. I how do I get my camera to send the signal through the hot shoe to make the flash go off?

Thanks in advance for answering.

PS. It's a manual flash, how would I use a manual flash with my Nikon. There must be a way because why would anyone buy it. How do you trigger them off?

You also have to put your D5100 into manual mode, then calculate the aperture setting/ISO/shutter speed (usually 1/60sec) to get the correct exposure.

Manual flashes are rather advanced operation, and not for everyone.

Before you put that flash anywhere near any modern camera, make sure that its trigger voltage is not more than 6 volts. If there's no reference in the handbook to the specific trigger voltage check it from the manufacturer's website. If you can't find the info then assume that the "trigger voltage" (not the battery voltage) is too high to use safely on your camera. If high trigger voltage hits the circuits of that camera it will very likely break down and you will be hit with a very big repair bill. YOU HAVE BEEN WARNED

Frankly a flash with a name like that must come from a backyard factory, and thus it may not be good at all.

You need to have the camera in manual mode. The Yongnuo flash needs to be in manual (M) mode too (it has slave modes, S1 & S2 - make sure neither of these is selected).

For a test shot, with the camera in manual, set ISO to 200, aperture to f8 and shutter speed to 1/160. Set the flash output power to 1/8. This should give you something to work with as a ball park exposure. If it's too bright then decrease the flash power or stop down the aperture, if it's too dark then increase flash power or open up the aperture.
If you want more ambient light then slow down the shutter speed. Don't forget your shutter speed needs to be at or below the max sync speed (usually around 1/200 sec, not the 1/60 sec that AWBoater said).

Edit: Take no notice of JohnP's dire warnings - it is a perfectly safe, modern digital flash. The high trigger voltages are only found on older flashes designed for film cameras.
Sure, it may be made in a Chinese 'backyard factory' but having owned & used them for a couple of years I can confirm their robust build quality.