While you can manually administer AWS, it is easier to do so with Terraform. Plus, it is best to follow Infrastructure-as-Code (IaC) practice.
You can find all the code snippets below (and more) in the following Git repository: https://github.com/whyn07m3/terraform-examples/tree/main/aws
CloudWatch + Simple Notification Service (SNS)
The configuration below will setup a CloudWatch billing alarm that will get triggered when estimated monthly charge goes over $10. When alarm gets triggered it will send an email to the specified email address. Before receiving any billing alarm emails, you will first have to confirm the subscription, which AWS will send you upon creating this subscription.