That device doesn't have much in the way of "Internal" storage, and much of the listed amount of storage would be taken up by the OS as well as any apps that came with the device. Add in any photos taken, contacts added, apps that keep a cache of things on the device, and the storage can be eaten up really fast. The only way to free up space would be to get a SD card for it and move things like photos, videos, music, (not apps), etc., over to the card to free up space. Then clear out the cache of things like your browser, facebook, etc., which add way to much junk to the device over time. And uninstall any apps (not just remove the icon but uninstall them if the phone will allow) ones you never use.
Since your phone runs an older version of Android, you wouldn't be able to convert the card into "Internal" storage to be able to run apps off of it. However, if it is possible to upgrade the phones version to a newer one (6.0 or greater) through the phones update options, then you 'might' be able to take advantage of that option. No guaranties though. Some manufacturers restrict that option on their devices.
First, you need to be using a phone that is running at least Android "Marshmallow" (or newer) or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your phone, or they don't work for you, then you have a phone that either has an older Android version or has been restricted by the manufacturer.
How to turn the "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.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the phone, some things may not work correctly, so make sure you do the reboot.