Unless your phone is running Android "Marshmallow" you will not be able to download apps directly to the card. You can move things like music, photos, videos, etc. over to the SD card, which will free up a lot of room on the device itself. I would also recommend backing up all those things to a computer or cloud account. So you don't accidentally delete them or lose them if the phone dies or gets lost/stolen.
Now if your phone is running, or can be upgraded to "Marshmallow" then you can use the instructions below to make the SD card the default download location for all including apps, as well as move apps over to it. However, and important note, to do this you must start with a blank SD card. Why? Because when the card is formatted...