It's always best to make a backup before you begin so that if anything goes wrong, you can restore using the backup.
If you're just worried about your personal stuff like browser history, you can use bleachbit which will overwrite the data.
sudo apt update
sudo apt install bleachbit
Run bleachbit as a regular user.
From the main screen of bleachbit, you should probably just select everything on the left menu. Note that it should delete all your browser saved passwords so be sure to back up those profiles before you begin.
Certain things like APT and system files need root privileges so just select everything and if it tells you that you need root access for whatever, you can adjust accordingly.
Also, make sure to click EDIT and then select Preferences and then make sure the option "overwrite contents of files to prevent recovery" is selected as shown in the following screenshot.
You may also be interested in overwriting your /temp directory and your swapfile. For those you could probably use a tool like secure-delete but just keep in mind that doing so is more dangerous so you should always be careful when running overwriting tools as root. Asking a separate question about that specific use (overwriting /temp and/or your swapfile using secure-delete) may yield more answers.
Another thing to consider is overwriting any free space on the disk in order to overwrite files that have been deleted that still exist in physical form on the disk. To do this, you will need to run bleachbit as root.
Running bleachbit as root is a bit more dangerous so be careful.
Run bleachbit as root and again, select everything on the menu to the left.
Also, make sure the option to overwrite contents of files to prevent recovery is selected.
