If you don't pay a monthly subscription, you are on the free trial, so go to the Square Enix store (make sure it's the store for your region) and buy the Complete Edition.

If you do pay a monthly subscription, you are not on the free trial so go to the Square Enix store and buy Shadowbringers.

Neither of these will include the upcoming expansion, Endwalker, which releases in a month. You could buy Endwalker instead of Shadowbringers, but then you won't be able to access red mage for a month.

If you don't pay a monthly subscription (on the free trial) and want to get Endwalker instead, you can buy the Starter Edition and then Endwalker.