Less is the comparative of little (used especially before uncountable nouns).
I earn less money than a clerk.
Fewer is the comparative of few (used before plural nouns).
I’ve got fewer friends now than I used to have.
Less of and fewer of are used before determiners (like the, my or this) and pronouns.
I’d like to spend less of my time answering calls from strangers.
Before nouns without determiners, of is not used.
If you want to lose weight, eat less food.
Lesser is used in a few expressions to mean ‘smaller’ or ‘not so much.’
the lesser of two evils
a lesser-known writer