It depends on what kind of files. If it is just photos, music, videos, etc. (not apps) then you should be able to locate on the phone a file management app. If not you can download and install one. Then use this to move the files over to the card. Or, my personal fav, is to connect the device to a computer via USB cord and move the files over to an empty folder on the computer, then copy them back over to the card via the same USB connection. This is faster as you can move all at once and you can then leave a copy on the computer as a backup.
To have future files save to the card you would have to go into the individual apps that you use to obtain (browser, etc.,) or create (camera, etc.,) the files and change that apps settings to save to the SD card rather than the internal storage.
Now, please note, before doing any of the above, if you are planning on trying to change it so that the card can accept apps (some phones will/some won't) then you need to not do any of the above first, as that would all be wiped when the changes below are done.
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.