A page at Amos deals very well with diminishing marginal utility. They even have interactive tables!
Chapter 4 of Price Theory by David Friedman deals with much the same thing.
I recommend reading both closely and taking notes of the key points.
Obviously 'utils' are a learning convention. Nobody believes utils are an effective measure of marginal utility in the wild! There are no units in economics. That's why it's not a hard science.