What happens when I restore my iPhone 5?

On my iPhone 5, a lot of my songs are cluttered and just messed up so I wanted to restore my iPhone and start fresh with putting music on it. If I restore it, will my phone number and critical stuff like that be erased or just trivial stuff like songs and podcasts?

All of contacts and everything will be erased you need to re dowload everything

Media files synced from your computer aren't backed up, but can be restored by syncing with iTunes. More info @ source.

Yeah, all your data will lost. Therefore, you can backup your files before restore your iPhone. Here is the guide about iPhone backup http://www.any-data-recovery.com/topics/mobile-devices/extract-iphone-backup.html could help.