Don’t back up your entire iPhone to iCloud. Loading it up with music you have downloaded from iTunes is a waste of space that could otherwise be used for essential data, things you can’t afford to lose.
Instead, back up your entire iPhone, iPad, or iPod, by connecting it to your Mac, opening iTunes, and clicking on your device. Under Backups, choose ‘This computer’ and select Back Up Now. This will save the data from that device on your computer, leaving the cloud free to take care of essentials such as Contacts, Calendars, Notes, and Emails.
To check what you are currently backing up to iCloud, on your iDevice, go to Settings.
Scroll down to iCloud, and look at what categories you have turned on. The setting of Documents & Data is important here. At the bottom, select Storage & Backup and then Manage Storage to see a breakdown of the largest data hogs. At the bottom of this window, you can change your storage plan if you need more space on iCloud.