storage on galaxy s5

Open your camera app tap the gear. Tap gear again and scroll down and change storage.

How do I get the pictures already on my phone from the device to the
storage of the SD card?

If you open Gallery app in 'settings', 'application manager' you will have the option to move pictures to sd card.

If you don't use DropBox, I recommned it to automatically upload my photos when I connect to my wifi. They can be managed from any device. It's been a life saver. My last phone had a faulty sd card I gave up using it.