Go to Settings>General>Usage>Storage and check to see how much storage you have used.
If you are deleting apps and other content, something about that message doesn't seem right to me. What are you trying to do when you get this message? Take photos? Shoot a video?
Try quitting the camera app completely and restart the iPad. See if the message goes away.
Go to the home screen first by tapping the home button. Double tap the home button and the task bar will appear with all of your recent/open apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar. Restart the iPad.
Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.