Before connecting your iPad to iTunes on your computer, go to iTunes in the toolbar and tap on Preferences, then select Devices from the menu at the top of the pop up screen. Once there, check. "Prevent iPods, iPhones, and iPads from syncing automatically." This will prevent syncing as soon as you connect to iTunes.
On a Windows computer, click on the rectangle icon at the very top left of the screen and select preferences from the pop up menu. The rest of the procedure is the same as on a Mac.
After connecting your iPad to iTunes:
If you're using a Mac you go to the iTunes toolbar at the top of the screen and tap on File, then select Devices from the pop up menu, you have the option to transfer purchases from your iPad. This will load the iTunes content from your iPad to the iTunes library on your computer.
On a Windows computer select the File tab in the iTunes toolbar and click on it. The same options are available as on the Mac.
When you connect to iTunes one of the options is to backup your iPad. If you create a new backup you can restore from that backup. Do this as soon as you've transferred your purchases from the iPad.
When you sync the iPad, you can choose to restore your iPad and set it up from the backup you just made. The content you had on your iPad will be loaded from the content in the iTunes library that was put there when you transferred your purchases from the iPad. This should stop you from losing any content or data.