Although you can unlock the vehicle with the app, we always recommend having a contactless card as a backup, which you simply need to tap on the reader on the windscreen to unlock and lock the vehicle.
You will have selected this card when you initially signed up and it will either be a contactless bank card or Oyster travel card - don't worry, no payment will be taken. Once inside the vehicle, you will find most models have USB connections for phone battery charging.