Before you can backup your iPhone from iCloud you need to make sure you have enabled Backup. To do this, go to Settings, then iCloud, then Backup & Storage on your iPhone. From that point, backups will happen automatically every day when your iPhone is connected to the internet, to a power source and has a locked screen. You can also backup your iPhone to iCloud manually by going to Settings, then iCloud and then Storage & Backup.

It is important to remember that your iCloud account only gives you 5 GB of free storage. Purchases you make such as music, books, apps, TV shows and your photo stream don’t use that free storage. However, iCloud mail, documents and backups do count against it and keeping that information below the 5 GB storage limit is important if you want your backups to be successful. Otherwise, you will have to pay for additional storage. Keeping your mail cleaned out and getting rid of items you no longer want or need can help make sure you stay under the 5 GB limit. Other items that are backed up that can take up space are photos and videos in your iPhone’s camera roll, device settings, app data, home screen and app organization, iMessage, text, and MMS messages, ringtones and visual voicemail.

Source Support Apple