Just confirming it is this Galaxy J5 > http://www.gsmarena.com/samsung_galaxy_j5_(2016)-7869.php? If so it lists support for an SD card up to 256GB max. You will need a Micro SD card for this from any size of your choice under or equal to 256GB. It simply goes into a slot similar to the SIM card in the back of your phone when you take the back case off. There should be plenty of YouTube tutorials or online guides for this but it is quite easy to do.
If you already have a 8GB SD card like you indicated in your post, you can simply swap them over. But you might want to put them into a Laptop or PC first to transfer the old files over to the new one. Hope this helps