To make the smartphone secure, the first step is to uninstall all the unusable apps.
Do not download apps from third-party websites other than Play Store or App Store.
Use cloud storage to store your important documents.
Before visiting any website, check whether HTTPS is enabled or not.
Do not connect your smartphone to computer via USB in any internet cafe.
For chatting, use secure apps like WhatsApp, Signal, Telegram.
Don't forget to reset the smartphone once every 6 months. So that all unknown files are deleted.