Starting point for Riemann sum, Integral proof?

Suppose f is integrable on the interval [a,b] and Q is a partition of [a,b]. Suppose $\displaystyle \epsilon >0$. Prove that there exists $\displaystyle \delta >0$ such that if P is any partition of [a,b] with $\displaystyle |P|<\delta$ and S(P,f) is a Riemann sum then

$\displaystyle |S(P,f)-\int_a^bf|<\epsilon$

I'm mostly just looking for a starting off point, or a direction to go. Should I try proving that $\displaystyle S(P,f)=\int_a^bf$ for $\displaystyle \delta$ small enough? How should I define $\displaystyle \delta$?