1) Ensure you have an SD card installed in your phone. The internal storage on most devices is usually fairly small, hence the need for additional storage via the external SD memory card. Check your storage space by going to Settings ->Storage. There you’ll find the info about total space and what is available
2) Use your external storage for apps. To move apps to your SD card: Click on your MENU button -> Settings -> Applications -> Manage Applications -> Select an app -> Move to SD card.
3) Move ALL media files (music, videos, etc) to your external SD card so that your internal storage remains clear. You can do this by installing a file manager app (such as Astro or Solid Explorer)