Final answer:
The first step to configure Amazon Route 53 as your DNS service for an existing domain is to register the domain with Amazon Route 53.
Step-by-step explanation:
The first step when configuring Amazon Route 53 as your DNS service for an existing domain is to register the domain with Amazon Route 53. This involves purchasing the domain name through Amazon Route 53. Once the domain is registered, you can then proceed to create hosted zones and resource record sets that will specify how your domain should be mapped to various Amazon Web Services (AWS) resources or other targets.