Photography Question?

I'm a teenager and I'm thinking about saving up to buy a Nikon d750 camera, because rn I have a Sony camera but I don't like how it works, so can someone help me decide what lens to get? Because I have no clue what each different lens does so i don't know which one to get. I plan on using my camera for taking photos of my family, but also nature. Can someone help me?

Which Sony camera do you have, and what about it specifically do not like?

For general photography on a full-frame camera, a lens around 24-70 or 28-70 zoom will be a good choice. It's wide enough for a good deal of landscape work and interior group shots, and it's strong enough for closeups of people. It's a great focal length for social events. Which 28-70 or 24-70 to buy is another question.

Nature photography encompasses a huge assortment of subjects ranging from tiny bugs, to plants, to elephants. There's no one lens solution for nature. A nature photographer could easily use an ultra-wide zoom, a 1:1 macro lens, a 70-200, 100-400 and a few others.

Because you're young and just starting out (based upon your question), I would recommend not going with the D750. On the other hand, I've not seen a single example of your work to know if buying a D750 is what you actually need. A common mistake that beginners make is in thinking that a more expensive body will make your photos better. This is nothing more than marketing, and it's a lie. If you don't know what you're doing, owning a D5 or a $50,000 Hasselblad won't make your pictures any better. They'll be super sharp, yes, but that doesn't mean that they'll be anything worth looking at.

Based upon your stated needs, you won't' find a one-lens solution unless you go with a lens like a 28-300. Personally, I'd rather use multiple lenses with better performance and larger apertures than use an all-in-one lens. Go with a standard zoom in the 24-70 range and a 70-200 lens combo. These two lenses will cover the vast majority of your needs.

If your Sony isn't doing what you want it to, it's probably YOUR fault.

Even if it's a bridge camera or a compact, you should learn to get the best out of it before you go any further. Certainly lashing out on a full-frame body and a compatible lens if you don't know what lenses do would be a catastrophic mistake.

Learn focal length. The d750 is a full-frame camera. That means that the focal length that is closest to what your eyes see is 50mm. We call that normal or standard focal length. If you go lower, you see more of the scene but the subject becomes smaller and we call that wide angle. If you go higher than normal, you see less of the scene but the subject becomes bigger or seems closer and we call that telephoto.

There are varying degrees of focal lengths to suit different shooting scenarios. Shooting people almost always require a normal/standard lens or a short telephoto for portraits. That lens is also good for nature. If you want a versatile lens to cover the most common shooting conditions any photographer will encounter, get a standard zoom lens. For the D750, that would be around 28-70mm. If you can find a 24-80mm, that would be better.

About lenses: Look at the metadata in the photos you take with your Sony camera and see if it quotes "full-frame 35mm" equivalent focal lengths in the photographs you have taken. The you will get some idea of the lens focal lengths you need to get for any full-frame digital SLR camera.

For general family snaps you need a zoom of around 28mm to 70 mm. For nature (and sports) you need much longer focal lengths, say from 150 to 500mm (a big, heavy, expensive lens). For small creatures you might need a macro lens of around 100mm, to be able to photograph ants and small moths close up.

I'd buy an entry level Nikon with a 18-55mm kit lens. That is ALL you will need for YEARS to come.
You do NOT need a D750 . It will be more camera than you can handle or need.
Maybe something like a Nikon D3400 ?
Refurbished are cheaper and sometimes better than new.

The Nikon D750 often comes with a 24-120mm kit lens. That would be an excellent lens to start with.