Show that the inner product of the Legendre polynomials and $p\in L^2 ([-1,1])$ is equal to zero if the polynomial has degrees less than $n$

Question

Let $(P_n )^{\infty}_{n=0}$ be the Legendre polynomials in $L^2([-1,1])$, normalised as $||P_n||^2=\frac{2}{2n+1}$.

I need to show that, for any polynomial $p\in L^2([-1,1])$ with degree less than $n$, we have $<P_n ,p>=0$, but I have no idea where to start with it (I would not like a full answer, but would appreciate a point in the right direction).