Three metaphors that spring to mind:
A stepping stone: something that aids you in getting towards your goal, something that helps you move forward
Example: I'm not your stepping stone/ You're trying to make your mark in society /
You're using all the tricks that you used on me /
You're reading all them high-fashion magazines (The Monkees)
The key that unlocked [doors] (can specify which door, or even substitute another word): the essential tool that enables an achievement
Example: The discovery of DNA was the key that unlocked the human genome.
The wind in (someone's) sails: A force that drives someone forward, giving them the energy and enthusiasm to accomplish a goal. Something can either put the wind in a person's sails (helpful) or take the wind out of someone's sails (thereby taking away their enthusiasm).
Example: The good grades Samantha earned that semester put the wind in her sails, and she's been studying diligently and getting high marks ever since.