How do I save to an SD card?

I got an SD card because my storage was getting full, but when I try to get apps, its says my device doesn't have enough storage. The SD card has nothing saved on it. I thought maybe I just can't put apps on their because formatting and stuff. Even if that's true, I still can't get images, downloads, or audio bits to save on it. If it didn't show up in SETTINGS->STORAGE, I would be worried that my device didn't even know it was there.