I don't know how to get my Yongnuo flash to work on my Nikon D5100? - 1

I bought a Yongnuo 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?

Some Yongnuo flash units have specific contacts for various camera hot shoes.

Some are only suitable for using in the full manual mode, they of course cost less and lure those looking for a "deal" to buy them, not knowing what skills are needed to use a flash using manual controls

I always suggest that people just save their pennies until they can buy the dedicated flash units designed by the maker of the camera the use.

You may want to return the flash and use the built in flash until you can budget for a flash like the Nikon SB-700

Is your camera out of auto mode?

Is the Yongnuo flash version you have designed for Nikon iTTL?

What flash model do you have? Is it specifically for Nikon? Yongnuo makes the same flash for different camera brands.

Without much to work on, set flash to Auto or Manual mode at ISO 400. Set camera to Manual mode, ISO 400 and to the fastest flash sync speed (1/125s is safe). Pick a target and get/estimate distance. Check the back of the camera for recommended aperture size at ISO 400 for estimated distance. Set aperture size on camera. Take a shot. If flash is too strong, you can step back, lower ISO on camera, or raise f/number. If too weak, do the reverse.

Keerok - the Yongnuo flash doesn't' have an aperture/distance scale on the back, it's not an Auto thyristor flash!

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) It would help if you specified WHICH Yongnuo flash model you have.

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).