The free GarageBAnd version that comes with a new iPad, only has a starter set of instruments, Keyboard, Drums, and Smart Guitar. You can buy aditional instruments with an in-app purchase, see this document:
Expanding GarageBand for iOS 2.0 content with an in-app purchase
Did you already buy an earlier version of GarageBand on a different device? Then see the last paragraph:
If you previously purchased an earlier version of GarageBand for iOS, you will get the In-App Purchase content for free. To restore your purchase, you may need to tap Already Purchased when you see the in-app purchase in GarageBand.