To pop open your exterior fuel door, simply pull up on that lever and then release it. This will disengage the spring lock that keeps your fuel door in place and allow you to access the gas cap.
From there, simply exit your vehicle, swing open the fuel door, and unscrew your gas cap. This will give you direct access to your gas tank and allow you to fill your car up! Just be certain that your car isn’t running when you do this, as that could cause a fire.