Could you please help with this?

Let $\displaystyle P $ be a non-empty partially ordered set and assume that no element of $\displaystyle P$ is maximal. Use the Axiom of Choice to show that there exists a function $\displaystyle f: \omega \rightarrow P$ such that $\displaystyle f(n) < f(n^+) $ $\displaystyle \forall n \in \omega$