Your device is out of memory, so you need to clear up some space.
First, go to your settings menu, then select "Storage". From here you can see how much space is available on your device, and what's taking up the most space. You're probably going to need to go through your apps and delete extra apps you don't use. You can do this by selecting "apps" from the storage menu, then by selecting an individual app from there.
Be aware, many apps that came with the device can't be uninstalled, as they're needed for the android system.
If you need help with anything else, check out the troubleshooting page.