Themes can quickly consume up storage space because themes can include images, sounds and icons. Remove themes you're not using. See
Deleting a theme.
Keep only the most recent data in your apps and files that you often need to use. You can back up your other data and files. For some tips, see
Ways of backing up files, data, and settings.
Uninstall apps that you've downloaded if no longer need them. Preinstalled apps can be disabled when you're not using them. See
Disabling an app.
Third-party apps that you've downloaded and installed can be moved to the storage card if you've set up the card as internal storage. See Moving apps and data between the phone storage and storage card and Moving an app to the storage card.