Use Firefox as your main browser. Chrome and Edge employ highly invasive tracking systems.
Install an Adblocker and generally refuse all non-essential cookies.
Use Signal as your messenger, it’s fully encrypted and well designed.
Never allow any commercial app to access your contact list.
Use a VPN when doing sensitve stuff online.
Be aware of the extensive telemetry of commercial operating systems like Android or Windows.
Create and maintain a shadow persona for your online activities, with an inspired name, corresponding email, random birthdate and disjoint interests to your own.
Avoid using any free Google or Facebook products if possible. Unwarranted user profiling is a serious threat to our democracy.