Using a file browser app, copy your photos from the internal storage to the SD card. After the copy is successful, delete the photos from the internal storage.
Welcome to Android Central! I would also put a plug in for Google Photos Auto Backup: https://support.google.com/photos/answer/6193313?co=GENIE.Platform=Android&hl=en. Remember that your phone's internal storage, as well as SD cards, do not last forever. If you have precious photos stored there and not backed up anywhere else, you'll lose them if something happens to the phone or the card.