Installing Tails OS
Get a USB stick you're ready to completely format and erase.
Get a SD card for file storage.
Goto https://tails.boum.org/ on your Online Computer.
Follow the installation instructions for your OS (I recommend the torrent link).
Restart your machine and boot into the Tails USB.
Note: While unlikely, you should verify your Tails OS ISO image. Follow the tails verification steps on a linux machine. Here's the link to the steps: https://tails.boum.org/install/expert/usb/index.en.html