All rides are paid from your account balance, which is loaded through your Balance Auto Update plan. The amount you selected when you first signed up for the app will automatically load when you start your first ride. That same amount will reload your account balance when it reaches $0, this can happen multiple times when you have a high riding day. This reload will transact automatically from your default payment method, which you can change in the Payment screen in the app.
If you have a new rider coupon, it will automatically apply to your ride before your account balance is used, but you will need to select a Balance Auto Update Plan prior to taking a ride.