Every time I try to change my wallpaper on my iPhone 5s it completely shuts down?

It's pissing me off I've had my phone for 8 months now and my phone crashes nonstop when I go on certain apps and people say to delete the app and install it again but I can't do that because my phone ran out of gb it had 16gb the ******* guy worked at the company where I got my phone didn't even ask how many gbs I wanted he just ******* gave me the 16 and I can't take pictures I can't get apps and I can't download music I'm on an 18 month contract and my phone has been acting really stupid lastly and it frustrating I've tried to restart my phone by pressing the lock button and shutting it down and that's not helping either I really want to get another phone but I have to deal with this phone I thought it was perfectly fine before but now I want to throw my phone at the wall please help

Kkkk