Remove the mechanical key from the fob (it's easiest just to take if off your key ring), and insert into mechanical lock hidden by door handle. The door will open and you'll be able to then open the engine compartment and access positive and negative terminals to charge your battery (the battery itself is hidden). If you do open the door and the interior light does come on, be aware that the alarm will sound unless you start the car. Depending on your model year, that means either inserting the fob or removing the dummy key. If you're not confident with any of this, it's time to call out your roadside assistance or a local specialist for some help.