Why does my phone say I have no storage left when I should have 8 gigs?

For the past few weeks, my phone has been refusing to let me download any new songs, apps, or take any photos because it says my storage is full. When I look at my storage settings though, it says I should have like 8 gigs left. I don't know what "other" is and why it's taking up so much space or how to clear it, and it's really frustrating to deal with because I love taking photos but it won't let me. It's also had some trouble connecting to cell service, and I keep having to reboot it to get a good connection. I have had this phone for almost two years now. Does anybody know why this is happening or how I can fix this?