Yea, you really don't want to install your own DB on here, you want to use their RDS instances for anything DB related. They are pre-installed, fully managed, Database instances. Makes life easy
If you did want to install your own DB (and why would you for a "getting started" bootstrapped product?) then you need to pick an instance type that works best for that specific use-case.
t2's (and AWS in general) are great, cheap, little virtual servers that can serve up your bootstrapped products web traffic without issue.
AWS makes it trivial and cost effective to scale up when you need to, but stay cost sensitive when starting out.
At least, that's been my experience...
You can ignore the whole CPU credits thing (and the likes) under this use-case, t2's provide a base line that is sufficient for almost all needs when you're bootstrapping a product.
I've found that you need to pay attention to that stuff when you are "pushing the envelope." A problem we all hopefully have at some point
My experience with them has been scaling up to over 100k visitors/day on AWS instances without issue.