Prove that if $\displaystyle A$ is transitive and $\displaystyle A \ne \phi$, then $\displaystyle \phi \in A$.

I can only say that if $\displaystyle A$ is transitive, then $\displaystyle \forall a(a \in A \longrightarrow a \subseteq A)$. How would I have to do in order to continue from here?

Thanks in advance.