wet behind the ears

Oxford Dictionary definition:

Lacking experience; immature

Example sentence:

On no grounds can what he is alleged to have done be justified though he looks still wet behind the ears for a hacker.

feather one's (own) nest

Oxford Dictionary definition:

Make money for oneself in an opportunistic or selfish way

Example sentence:

Politicians will stoop to any level to feather their nests and retain or regain power.

pull one's socks up

Oxford Dictionary definition:

Make an effort to improve one’s work, performance, or behaviour

Example sentence:

It is time the trade unions in the education sector asked their members to pull up their socks without playing the blame game.

nip something in the bud

Oxford Dictionary Definition:

Suppress or destroy something at an early stage

Example sentence:

All airlines must carry out frequent psychological assessments of their flight crew to nip any behavioural problems in the bud.