If I reset my iPhone 5C back to the factory settings, and afterwards sign into itunes, will all my music automatically come onto my phone again or will I have to sync it with my computer to save my music?
iPhone 5c, iOS 8.3
Better do a backup first - I'd never do a reset without one.
Basically, you just backup your device as suggested by babowa and then restore it to factory settings. Once you do this, you will be able to sync your music back to your device.
Take a look at this:
Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support
Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support
Resetting iPhone 5C
