You have only one pet. This pet is given to you at the start when you get your Soul Crystal. Your weapon is a Whip. From the start every quest is you bonding with your pet while also gaining other pets(off gcd skills) to help you in battle.

As for how it'd work as a tank, you wouldn't be able to move the pet like you would a SMN or SCH or MCH. Instead, you'd have stances. Your Offensive Stance has the Pet to your side and your Defensive Stance has the Pet in front of you.

When in Offense, your pet is the main attacker. Your abilities are using your whip to rouse and command your pet into action.

In Defense, your pet is taking the Brunt of the Damage, it does not have a health bar of its own and instead just lessens the damage you take by being your 'shield'.

Your Off Global Cooldown attacking or active defense skills would be calling forth other creatures to strike or protect.