First of all, if this is not the computer that you sync with, your iPad will be erased during the update process and if you do not have a backup on your own computer, you will be starting all per again with a new iPad. You will lose everything on the device.
read the section about using iTunes.You should read this as well.iOS 4: Updating your device to iOS 5 or later.
If you have the iPad 2 - you will only be able to update to iOS 6.0.1 now. The original iPad can be updated to iOS 5.1.1 and that's as high as you can go.