Said when someone has expressed a desire or opinion, meaning "That viewpoint is not unique to you". It can suggest that the speaker should stop complaining since many others are in the same position.
Example: "If this train doesn't come, I'll be late for work!" "Join the club!"
If you jump in feet first, you approach a task or activity with little to no hesitation. When you jump into a swimming pool, there's no going back once your feet leave the ground- you will enter the water. When you jump in feet first with a new activity or task or requirement, you give it your best, knowing there is no going back.
Said of a salient point in a television show or other activity at which the popularity thereof begins to wane: The Flintstones jumped the shark when a man from outer space came to visit them. The expression derives from an episode of the television sitcom 'Happy Days' in which Fonzie, clad in leather jacket and on water skis, jumps over a shark. That episode was widely seen as the beginning of the end for the formerly popular series.
Things, especially education, that affect and influence us in our childhood shape the kind of adult we turn out to be. (There are various versions of this, like 'As the twig is bent, the tree's inclined' and 'As the twig is bent, so the tree inclines', 'As the twig is bent so is the tree inclined')