Come out of (one's) shell American informal
They use the image "snail" or "shellfish" because they have the shells to protect them.
To overcome the shyness or unconfidence to show oneself or to become sociable
A new job is so good because it can help her to come out of her shell.
When we are enough 18 years , we have to come out of our shell and step out of our comfort zone.
The verb "come" should be conjugated according to its tense.
The source said that It is written by Ebenezer Brewer in The Ancient Sage, 1885.
Children or adults often say this phrase to get their friends to join in something, especially to jump into a swimming pool (pond, lake, etc.)
“Last one in is a rotten egg!” yelled Jane, jumping into the pool.