When your contract application is accepted by the network, your billing date (or Direct Debit date) will be confirmed on your first airtime bill. But the quickest way to find out is by downloading your network app (links at top of page). Alternatively, you can contact the network directly to find out.
If you receive your phone before your network billing date, you get given a partial allowance so you can start using it straight away. This partial allowance is chargeable, and will appear on your first bill.