Which is a better Lens to Start off with a Nikon D750?

Nikon 50mm f/1.4
Nikon 24-70mm f/2.8
Nikon 24-120mm f/4

24-39 mm

Nikon 50mm f.1.4
The normal 50 mm lens is always he best material to sart with. You have a sharper image, Better light with lower price o practice with

It depends upon the subjects you tend to shoot

The 24-70 mm f/.28 is one of the lenses most full frame shooters start with. The second is usually the 70-200 mm f/2.8. With these two lenses you can shoot just about everything from group or team photos to shooting basketball from the baseline indoors to shooting just about any other sport where being able to reach out into the field of play, the 100 to 200 mm range excels.

The 70-200 mm f/2.8 extends the shooting range and that lens is used a whole lot when shooting sports indoors or under the lights.

The f/4 lenses are typically too slow if you want to shoot indoor sports.

The 50 mm f/1.4 lens is rarely used on full frame cameras (but is an amazing lens for cropped sensor cameras when used as a portrait lens. If you plan on shooting portraits, then the 85 mm f/1.8 would be a better choice.

The 24-120 is a notoriously bad lens for the price. The 28-70 f/2.8 is vastly superior to anything else on your list and would make a great first lens. To see how this lens stacks up against others, go to http://www.photozone.de/...e/nikon_ff

50mm if you want to learn composition aside from exposure or when you see yourself shooting more in low light. The fixed focal length will force you to walk around to get the right frame.

24-120mm if you want more flexibility in shooting landscapes and large group shots to people, pets and most anything to headshots to details just out of your reach.

24-70mm if you want something in between the above two.