Prove that $\mathcal{O}_K$ for $K = \mathbb{Q}(\sqrt{3})$ is euclidean

Question

I'm having trouble with this question concerning the ring of integers of a real quadratic field:

I know that if we can prove that the ring is euclidean then every ideal is principal, and thus $C(\mathcal{O}_K) \simeq 1$.

I just don't get what the question means when it says "the absolute value of the norm fuction" and how can this help the proof that $\mathcal{O}_K$ is euclidean