If however your #security depends on not only you not reading their posts, but them also not being able to read yours, i.e. like Twitter's private accounts, you'll need to change some settings.
Not only do you need to set your account to private to control who follows you (preferences->profile->lock account), but for each post you need to make sure it is set to followers-only.
But generally, social media is a public affair. Don't post anything you wouldn't want your employers, your children or your parents, your government, your insurance or strangers to see. This includes answers to security questions, so don't do those eighties band name memes.
On Pleroma the settings are similar: select the lock instead of the globe from the scope icons when posting. Like in Mastodon, doing this without locking your account makes no sense, so you even get a warning. In your profile settings you can lock your account and set the default posting scope to followers only.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!