How to take a picture like this?

So i'm a photographer in training & I've seen a few pictures taken at a certain angle which would be very cool to do, however I can't quite figure out how to do it.
These are the pictures below:

(I have a Nikon D5100 just in case that would be helpful)

Hmm, it doesn't look too difficult. Just have your subject facing directly into the mirror while the camera is off to the side and angled into the mirror.

Basically the camera is focusing on the reflection of the subject in the mirror. To do this focus on the subject while not reveling the camera in the mirror. It does not matter what camera you use.change the settings to manual focus and focus on the mirror.

Stand behind the subject, point the camera over the subjects shoulder and focus on the reflection.
Play with the angles until you are not in the shot.
And no, the camera has nothing to do with it. It's all about getting the angles right.
And make sure the mirrir is spotlessly clean. Or you could write something in lipstick in one corner in an arty way.

I'm thinking you might want to do a self portrait this way?

If that is the case, you need your camera… A tripod… And a shutter release cable. These shots are usually pretty difficult to get… It takes a few rounds to get your own expression, the composition and the lighting right.

You'll want to have your camera on manual focus. Have a friend or an inanimate object be in the place where you will be standing. Focus on it's reflection. Move the inanimate object and replace yourself in the photo. Adjust your expression to how you want it and push the shutter on the cable.

Like I said… It will take a while to get in the flow, so don't give up! Self portraits are incredibly difficult.