Show that a group of order $n$ is isomorphic to $\mathbb{Z}/{n \mathbb{Z} }$ if and only if it contains an element of order $n.$

How should I define a function from $G$ to$\mathbb{Z}/{n \mathbb{Z} }$ to show that it's a bijective homomorphism. We know that elements of $\mathbb{Z}/{n \mathbb{Z} }$ are $[0]\_n,[1]\_n, \cdots , [n-1]\_n$ and it's an (abelian) group with operation $+:~[a]\_n + [b]\_n=[a+b]\_n.$ Any help is much appreciated.