How do i take 12 dark, 12 light and 12 perfect pictures with 400 black and white film on a vivitar 3800n camera?

For normal exposure, set the ISO dial to 400. Meter normally, adjusting the shutter speed and/or lens aperture until the green LED in the center lights up.

For dark pictures, you want to underexpose about two stops. One way is to meter as above, then stop down the aperture two stops (larger F/numbers) or increase the shutter speed two stops (larger number) or a combination of the two. Alternatively, set the ISO dial to 1600 and meter normally.

For light pictures, you want to overexpose about two stops. Meter normally then open up the lens two stops (smaller F/numbers) or slow the shutter speed two stops (smaller numbers) or a combination. Alternatively, set the ISO dial to 100 and meter normally.

You can also experiment. Over/underexpose one stop, then two, then three, then four, and even five. You might want to take a notebook with you and write down what you're doing. Record both the setting and how many stops over/under it is. When you get the film back, you can refer to your notes and see what happened.