Let's first get this out of the way: Ubuntu does not need a lot of maintenance, don't expect magical performance boosts after cleaning up.
However you will notice a bit more speed and creating some hierarchy in all your documents and images won't be bad too. I would recommend to do this once a year. Now we got that out of the way let's dive right in...
First start by emptying your Downloads folder and your Trash. Both are common places for old files to bulk up really quick. If you accidentally throw away a file you want back while cleaning up your home folders in the next step you can find it back in the trash.
Secondly comes (probably) the most time-consuming step, tidying up all other folders located in your Home folder. The trick is to create some hierarchy so cleaning up next time will be a breeze.
Thirdly open the Ubuntu Software Center, hit the "Installed" tab at the top, go through all categories and remove all software you don't use.
Now we got that out of the way set let's move onto some commands that will clean Ubuntu for you, enter the following commands in this order:
sudo apt-get autoclean
Removes partial packages.
sudo apt-get clean
Cleans cached packages.
sudo apt-get autoremove
Removes unused dependencies.
Press Ctrl+Alt+T to open a Terminal window and press the Enter key to run a command.
Browsers tend to clutter up with history, cookies and other things. Clearing your browsers will delete all the mess you have collected from browsing on the internet. While in FireFox press Shift+Ctrl+Delete, then first choose for "Everything", check all boxes and click "Clear now".
Your Home folder could contain hidden folders from apps you deleted. So navigate to your home folder and press Ctrl+H to view all hidden files. Then look for folders with the name of old apps you don't have installed anymore and delete them, the same goes for the folder named ".config".
Hidden files will be prefixed with a dot.
That's it, your Ubuntu machine should be clean and lean by now.
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