Unfortunately that phone is running a system that won't allow (in most cases) downloading directly to the card versus the phone itself. You would need to be running "Marshmallow" to have that feature, especially with apps. Most apps won't run from a card either.
Now some apps themselves will let you download things used within the apps to the card, but not most.
Your best bet would be to move all things like photos, images, videos, music, etc., to the card. This will free up a lot of space, as they can be huge files. You might also want to consider backing up said files to a computer or cloud account.
Also, try cleaning up the phone. There are quite a few apps out there that will help you clean up excess things on the phone.