First, open iTunes on your computer and go to Preferences; the Preferences window will open up. Select devices from the menu at the top and check the box next to "Prevent iPods, iPhones, and iPads from syncing automatically", then click on the OK box at the bottom of the Preferences window. Now connect your iPad to iTunes and you should get a pop up that says there are items on your iPad that are not in your iTunes library. There will be an option to transfer purchases, take that option and anything on your iPad that is not in your iTunes library will be downloaded to the iTunes library on your computer. This will not erase anything on your iPad.
BTW, the reason you go into Preferences to stop automatic syncing is to stop iTunes from erasing the contents on your iPad and replacing it with the contents of the iTunes library on your computer, which will happen if the syncing starts automatically when your iPad is connected to iTunes.