First of all you'll want to authorise your new computer to use iTunes if you haven't already done so. To do this in iTunes select Store> Authorise This Computer and follow steps to complete, if you have more than one iTunes Account make sure you sign in with the one you made purchases on.
Next connect your device to iTunes, select File>Devices>Transfer purchases from...
Hopefully things should now be okay - if you get a message when it comes to synching that things will get deleted make sure everything has synched over from your iPad to iTunes and do it backup just in case - although depending on your backup method it should do this automatcally when you connect to iTunes, but you can confirm this by checking the Summary tab for your iPad. If all appears well go with it, and you should be okay.
Hope this helps.
Right now you have two different issues going on. I assume that you are getting a message that the "sync session cannot be started" or something to that effect - correct? If so, quit iTunes, restart your computer and restart your iPad.
The second issue is that you have to make the new iTunes library your syncing library. You have added content to the iPad that is not in iTunes on your computer so your computer's iTunes does not recognize your iPad as belonging to the iTunes library on the computer. When you sync with iTunes, iTunes will replace what is already on your iPad with whatever is currently in the library on the computer.
This will explain what you need to do in order to make this all work now.