You won't be able to purchase anything until you log back in
The first thing you’ll have to do with your new iPad is sync it with iTunes on your Mac. Once you do, your iTunes account will automatically be associated with the iPad and all purchases made from the App Store, iBook Store or iTunes Store will be with this account. If the wrong iTunes account is logged in on your device, or you plan on selling the device, you will need to log out of the iTunes account.
To log out of your iTunes account, open the Settings app from the iPad home screen.
Scroll down and tap on the Store icon in the left-hand column.
Once you tap the Store tab, the current iTunes account associated with this device will be displayed on the right-hand side as the Apple ID.
Tap on the Apple ID to bring up the options window. Here you can view your account details (billing information, etc.) and Sign Out of the account. Tap on Sign Out.
Once you’ve signed out of the iTunes account, you will not be able to purchase any songs, apps, or books until you sign into another iTunes account. Tap Sign In to add another iTunes account to the device.