The SD card is a great place for your music, photos, videos, and documents.
Copy (not move) them to the card, verify the copy worked correctly, then delete the originals from the internal storage.
For apps that do not let you pick where their downloaded files to, you may need to manually copy the pics/videos to the SD card and delete them from the internal storage every now and then.
The internal storage is where your apps should be (and stay).
Some apps that use large amounts of storage (camera, maps, GPS navigation, games) will allow you to move their data to the SD card while leaving the app on the internal storage. Check the settings in each app to see if they give you this option.