Check the disk space (may need to install ncdu first):

sudo ncdu /

Clean up unused stuff:

sudo apt-get clean
sudo apt-get autoclean
sudo apt-get autoremove

clean: clean clears out the local repository of retrieved package files. It removes everything but the lock file from /var/cache/apt/archives/ and /var/cache/apt/archives/partial/. When APT is used as a dselect(1) method, clean is run automatically. Those who do not use dselect will likely want to run apt-get clean from time to time to free up disk space.

autoclean: Like clean, autoclean clears out the local repository of retrieved package files. The difference is that it only removes package files that can no longer be downloaded, and are largely useless. This allows a cache to be maintained over a long period without it growing out of control. The configuration option APT::Clean-Installed will prevent installed packages from being erased if it is set to off.

autoremove: is used to remove packages that were automatically installed to satisfy dependencies for some package and that are no more needed.

See a related question on askubuntu: https://askubuntu.com/questions/3167/what-is-difference-between-the-options-autoclean-autoremove-and-clean