Preference relation that is represented by a utility function problem

I have a problem and i'm stuck already...

Given a preference relation $\displaystyle \preceq$ that is represented by the utility function u. If $\displaystyle \preceq$ is monotonic, show that $\displaystyle \preceq$ is represtented by $\displaystyle \bar{u}$, where $\displaystyle \bar{u}(0)=0$ and $\displaystyle \bar{u}(x) \geq 0$, for every $\displaystyle x \in \mathbb{R}^n$+ (n-dimension positive real numbers)

Can anybody help?