Be sure to read about pros and cons of setting up your SD card as internal vs external storage before making a decision to set yours as internal. I did some reading about this last year when I got an Android Marshmallow phone and decided to use mine as external storage - obviously you may have different needs. At the time I was looking into it, I found plenty of articles and forum discussions about the pros and cons via a Google search.
Depending on your apps, you might not be able to store files on an external SD card.
For example, Bookari/Mantano stores books in internal storage:
http://support.mantano.com/customer/...not-supported-
Apps such as your camera may have their own settings that allow you to choose whether the default storage location is in the phone's internal storage or on the card. You may need to do some more googling or refer to your phone's user guide to find out how to access these settings.
Here's a brief video showing how to move image files to SD card on a Nougat device (I believe this is for having the SD card as external storage) - it looks pretty simple.
https://www.youtube.com/watch?v=p1233Kqg450