I've had years of experience tanking in MMOs, and I have to say it is different than other MMOs tanking in FFXIV.
Just like others said, be open with your party that you are new to it. Be open to suggestions and trying new ways of doing it. It's like my FC motto: "Skill can be taught, attitude cannot".
If you have the attitude to excel and try your best, and be willing to research and learn new things, you already are a great player in my book. It really just comes down to stick time and learning abilities. I will share this from my low level experiences with Gladiator:
Until you get Rage of Halone, your best opener is going to be Flash > Flash > Fast Blade > Savage Blade. Sometimes opening with a Savage Blade without the Fast Blade to combo it is better. Just remember to mark before pulling, and tab through the enemies to make sure no one is too high vs you. Obviously you want the enemies on you, but definitely NOT the healer.