I want to use Manual Mode but I don't understand why, when I set my ISO to 100, I see in the picture settings that the ISO changed to 3200. How can I set it up to be what I set and not what the camera set? I was watching some videos but I don't have auto ISO on my D3100. Do I need to change to other settings?
You appear to have a Nikon camera. On all other brands, one chooses from a list of ISO sensitivities or Auto. The camera then uses the specified ISO for every option other than Auto.
With Nikon, if you have Automatic Sensitivity Control enabled in the Shooting menu, then the camera will use selected ISO as the default and actually select another sensitivity, up to the a user-specified maximum.
To change this behavior, simply set Automatic Sensitivity Control to Off.