The definition of make a mountain out of a molehill is:
(verb) to make something unimportant seem extremely important
Young children often make a mountain out of a molehill when they get upset after seeing their parents fight.
Our boss made a mountain out of a molehill when he fired Wayne for making a simple mistake.
A: Oh my god, my boyfriend just left me!
B: Oh, sorry Sarah!
A: My life is over! I will never love again! I’m going to kill myself!
B: Don’t you think you are making a mountain out of a molehill? You only dated for two weeks!