What phone do you have? How much memory is available?
If the phone supports an SD card, you can move photos, music, documents, etc to the card to free up space on the phone itself. You can also try going into the phone settings, app manager, and clear the cache for the apps.
The message you are getting means exactly what it says - there isn't enough memory on the phone to complete the operation. Delete unnecessary apps and files, clear cache, move files to an external location - all these will free up memory on the phone.