You have a statement saying something is true for all $n\in \mathbb Z$, so you know
- how you have to start: by saying "let $n\in\mathbb Z$ be some integer".
- how you have to end: by saying "therefore, $n^2\geq n$".
Now, you need a path from your start to your finish. I suggest you start by splitting the cases, since you know that either $n\leq 0$ or $n>0$. If you can prove:
- If $n\leq 0$, then $n^2 \geq n$
- If $n>0$, then $n^2\geq n$,
you are done. The first one is almost trivial to prove, and the second one is not much harder, since, if $n>0$, then $n^2 = n\cdot n = n + (n-1)\cdot n$.