With your iPad connected and selected under the Devices list in iTunes, visit each of the tabs at the top of the page (Apps, Music, etc) and set up syncing the way you want.
Two warnings.
Don't play with the Info settings unless you know what you are doing.
If you turn on App syncing, make sure every app currently on your iPad has a check mark next to it before starting a sync, or it will remove it. You can put them back easily enough, but you will lose any data or files stored in the app; and if you accidentally sync again while the app is missing you'll lose the ability to restore that data via the backup.