You can initiate this in two ways. The first is to connect your iPad to your computer, launch iTunes, select the iPad in iTunes, click the Summary tab, and in the Backups area enable the iCloud option.
The other is to choose to backup to iCloud from the iPad itself. You do this by going to Settings > iCloud > StorageStorage & Backup and in the resulting screen enabling the iCloud Backup option. An alert appears informing you that your data will now be backed up to the cloud. Tap OK to allow that to happen. Alles zu Storage auf CIO.de
Back up via your computer
To do things the old fashioned way, launch iTunes on your Mac and string your device's syncing cable between it and your iOS device. Select your iPad in iTunes, choose the Summary tab, and in the Backups area of the resulting window click on Back Up Now. If there are apps on your device that aren't in your iTunes library a window will appear asking if you'd like to back them up. The choice is yours as you can always wirelessly re-download any apps you're missing from the iTunes Store, but restoring them to your iPad will happen more quickly if they're being copied from your computer rather than being downloaded over a slowish broadband connection.
iTunes will now set about backing up any apps on the device that aren't in your iTunes library as well as other data the device contains.