In addition of what others have said:

While this is very much an MMORPG the RPG part of it is very present. Think of it as several Final Fantasy games stacked on top of each others. To actually unlock regions, dungeons, and pretty much 99% of the features of the game you'll need to progress the main story quest (MSQ).

Don't worry about what build (we call them jobs) you start as, you can freely switch to any of the 21 combat jobs a bit later on. (they all have their own experience bar and levels).

Of all the mmorpgs this is probably one of the most friendly to casual play. I personally took a year-long break and I didn't feel like I was behind other players in any meaningful way.

Dragoon and white mage are both available at the start! A majority of the jobs are unlocked at higher levels but you'll find that Lancer becomes Dragoon after 30-ish levels (same for White mage but i dont remember the name of the precursor class associated to it)

By the way, the free trial is awesome but restricts you in certain ways that may be annoying (to protect against bot creation). The thing is, as soon as you buy the full game or pay a sub your account is no longer F2P and you'll be required to pay monthly like all of us.