I recommend you go read through this site which will help give you an introduction to the fundamentals and a good foundation. In particular you should think about what your threat model is, since that will decide what measures and precautions you will need to take going forward.
Please also note that this forum uses English, so if that is problematic for you I also recommend using a translator like Deepl or Lingva to translate your questions if it is for you express yourself in Swedish?
@Mods: Just FYI, the title reads "Advice and tips to raise the device security and integrity to the maximum".