simple rule, if you don't want something viewable by others then don't expose it to the internet. Its not a complicated rule, however many people fail this simple bit of logic.
An example, family photos, holiday videos, music and tv shows. All things that don't really matter if someone gains access to. It's at most an invasion of privacy.
Another example, bank statements, birth certificates, financial documents, scans of your credit and debit card, IoT. These are all things that pose a potential risk to you if someone gains access to them. Don't put them on the internet, nobody can ever find them on the internet.
The internet by its very nature is built to share data, the easiest way to avoid sensitive data from being breached is to not have it on a device connected to the net in the first place.
Lets say you buy groceries but don't have a fridge, your next door neighbor offers to share their fridge as its massive, makes sense. However if you own a fridge, why would you use your neighbors fridge?