I need to show that if $A \subseteq B$ then there exists a function $f$: $A \rightarrow B$ that's injective.

I'm not sure how to start here. My knee-jerk reaction is to just give the definition of each (e.g., $\forall y(y\in a \rightarrow y \in x)$ and $\forall xy (x\ne y \rightarrow f (x) \ne f(y)$.) However, I'm not sure how this helps me.

Any advice on how to proceed?