Have you already fired up the AWS Cost Explorer (https://docs.aws.amazon.com/cost-management/latest/userguide/ce-what-is.html)? This will help you understand where your spend is highest and it usually will also make recommendations as to what you can do to bring costs down. Not sure where you are currently on your journey, but if you just started most likely you’re paying for what you use for EC2 and RDS (which AWS calls On Demand pricing). This is great at the start, as you do not have to make a long term commitment, but is sure to rack up your cost quite quickly if usage goes up. But the good news is that when you grow, you will have a minimum use that will always be there. For this, AWS allows you to reduce cost by making an upfront commitment (Savings Plans and Reserved Instances). This is typically suggested by the Cost Explorer.
Hope this helps,
I would also like to refer to a nice blogpost that my colleague Trong Tran wrote about managing AWS costs: Say Yes to AWS: 3 Tools to Manage Cloud Costs