Someone gave me a Sigma f/4.5 70-210 lens and it won't fit my Nikon D3200. Does anyone know what lens adapter I need?
It all depend on what mount the lens is produced for. You should check if the lens is produced for Nikon or not.
There is a lot of complexity to this answer. Without knowing which mount the Sigma lens was designed for, there is no way to know the answer. Additionally, it is most likely you won't be able to electronically control the lens so it will be fully manual. If you don't have an aperture ring on the lens, you won't be able to control the aperture for the lens most likely. Autofocus also won't function.
It is also entirely possible that you can't use it. If the lens is designed to sit further away from the sensor than your camera normally would put it, then an adapter should be possible, however if the lens is designed to sit closer, you are out of luck.