It should show on your account as being a pre-order : About iTunes Store pre-orders - Apple Support
When it's released you should receive an email, and if you don't currently have a payment method or balance on your account then you will need to add one so that it can be paid for (pre-orders are only charged for on release, not at pre-ordering). When it's paid for, if it doesn't download automatically then try the Store > Check For Available Downloads menu option on your Mac's iTunes or see if it shows in the Purchased link under Quick Links on the right-hand side of the iTunes store homepage for downloading (or on your iOS device if it shows in the Purchased tab in the iTunes store app for downloading on it).