What adapter do I buy for a sigma 75-300mm macro apo lens to use on Nikon d3200?

I do not know the camera it was used on before.

If it has a Nikon mount you don't need an adapter. If it has any other mount, it won't work on you camera.

Did you check it was for a Nikon before you got it, if you've already bought it?

Before buying ANY third party lens, you have to make sure it has a mount for your camera. You just can't buy a third party lens and "hope" it will fit on your camera if you just have the right adapter.

Look at the end of your lens. If it says "N", then it will fit. If it says anything else, it will not fit and you need to return it to the seller.

Find out which camera it was used on before, and if it wasn't a Nikon, leave it alone.

Even in Nikon fit, there are some things you need to know;

A manual focus lens will fit your D3200, but won't allow metering (only M mode will work, and you'll need some means to determine correct exposure) or autofocus. This also applies to any other mount with an adapter, and is more trouble than it's worth.

An early version AF lens without a motor will allow metering, but not autofocus.

Only a motorised (HSM - Sigma's equivalent of AF-S) lens will allow full functionality with your camera.

Unless you know the mount of that lens, you will never find the right adapter for it.

Take a picture of the mount and show us here.