Have you tried restarting the phone? Sometimes that is all it needs.
However if the problem of lack of storage continues, then you will have to move some things off of the phones internal storage. Either deleting them from the device or moving them over to an SD card. The quickest way is to just move photos, music, videos, etc. over to the card and that will free up space on the phone.
Also, you should make sure that nothing is running in the background. Just getting out of an app does not shut it off. It just keeps running and using resources. So making sure they are actually close is also important.