Well, if your phone is running Android "Marshmallow", a.k.a. 6.0, then you can try the instructions I will include below to see if you can use your SD card (if you have one that is) for apps. If it isn't running that, or can't be upgraded to it, and there is also the possibility that the manufacturer has restricted the option (they do that sometimes), then you are left with moving some other things.
Again, if you have an SD card, you can move things like photos, music, videos, etc., over to the card. If you don't have one, then you can just as easily plug your phone into a computer, via USB cord, and move all that stuff (music, etc.,) over to a folder on the computer. This is also a good way to do it, even if you do have the SD card to move them all to. Why? Because saving them all to a place other than a phone is safer. Phones get lost, stolen, erased, damage, die, etc., leaving you out of luck for all the personal stuff you have on it. This is why I always tell me people to back up their files. Always, and regularly.
It is also handy due to the fact that you can organize them info folders and rename them easily on a computer and then copy them over to your device (phone or SD card) and have them in an order that is easier to view/search. Then the copy leaves the original on the computer.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.