take up the cudgels

Oxford Dictionary definition:

Start to defend or support someone or something strongly

Example sentence:

He courageously took up the cudgels on behalf of the child combatants and strove to pressure the UN to adopt stringent measures against forcible child conscription.

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.