If you are new to AWS, you should know one thing - every service is paid in AWS. It sounds naive, but many people don't realize how expensive it could be in reality, and by default AWS will not notify you if your account generate too much costs (regardless if it was misconfigured or abused by 3rd party ).
So usually I follow one rule: Before using AWS create a monthly budget for your account.
Go to AWS Budgets
Create a Budget
Select a monthly recurring budget (please not that you could be still charged over this sum). Usually I set it to 10$ for in-house projects.
Click Next and on the next page create an alert (most important part)
It could be enough to have just 1 alert, for example when you reach 60% of a budget (6$) . Don't forget to add an email
You can have more than 1 Alert, but just having 1 will potentially save your money.
So having this notification , once you reached the 60% of the budget, you will receive an email saying that threshold is reached.
AWS will not stop charging you after reaching this threshold !
So you will need to login into the console and figure out what services are being used and needed to be cleaned up. Luckily it's quite easy to see what is using your budget.
Go to Bills and see the details