elementary set theory - Prove the countability of $mathbb Q times mathbb Q$ and $M_{2 times 2}(mathbb Z)$

Using the fact that $\mathbb N \times \mathbb N$ is countable, or otherwise, prove that the following sets are countable.

a) the set of all points in the $(x,y)$ plane with rational coordinates

b) the set of all $2\times 2$ matrices with integer entries.

I really don't know where to start... and I've never done matrices either!

I know that I need to prove there exists a bijection, but from there I'm lost.