Definitely set a CloudWatch alert to notify you if the cost goes above a certain limit. And make sure you check that email regularly. It takes a small misconfiguration to rack up a hefty bill. Best to catch it early.
Other suggestion is to stay away from the console or command line. Learn how to use CloudFormation or, much better, the CDK. You can set up a full serverless application stack, run it for a while, then delete the whole thing so it won't accidentally incur extra charges. It's also a good practice so you can include the stack setup code with the rest of your code.