Changing the aperture on my dslr?

I have a Nikon d3200 and recently got a Sigma 75-300mm lense When I attach it to my camera it will only work in manual mode which is fine… But I don't know if that is normal? Also when it is attached to my camera it won't let me change the aperture. Yes i did hold down the exposure compensation button and turn the command dial at the same time and it is not doing anything. Please help. I need to change the aperture my pictures are turning out way too dark. Also my flash will not work with the larger lense.

I'm not familiar with this specific lens, but I do know that when you use older Nikon lenses, you need to lock the aperture on the smallest aperture (largest number) to make them work with the command dial. Take a look at this picture and read the caption. Look for anything on your Sigma lens that might do the same thing.

If the Sigma lens has a manual aperture ring, for you to be able to change the aperture in the camera you need to set the aperture to its smallest setting. If you don't, then you need to set the aperture using the aperture ring and not in camera. Most of the time, lock it in to the smallest aperture and use the in camera settings. Where using the manual ring is useful is in things like macro photograpy when working with extension tubes, bellows etc where in camera aperture setting will not work properly