On any Android device, you need to keep a certain amount of free space on the internal memory for proper functioning.If your device has a memory card slot, you're in luck. MicroSD cards are cheap and most phones support cards up to 32GB in size.

You can also get larger cards if your phone supports them. Once the card is installed, you can start moving photos and videos. Next step is to move apps from internal memory to the memory card. To do this, get the free App2SD by Sam Lu. 

It clearly shows you which apps are movable and which ones aren't. If you don't want a bunch of apps, you can uninstall them all in one go (batch uninstall). It will also show you a list of apps that can be frozen (to stop them from eating up more space) and those that can be hidden (they'll disappear from the app drawer).