During Character Creation, you will have to pick a Class from one of three main Roles, Tank, Healer, and DPS. While this is a more impactful choice than your deity, don’t worry too much if you find yourself second guessing your selection. Around the time you level your starting Class to level 30 and get far enough in the main quest line, you will obtain a Soul Crystal (or Job Stone) which will essentially make Classes irrelevant. And you can make progress toward a new Class by getting a weapon associated with a Class different from what you have.
Unlike other MMOs, FFXIV allows leveling multiple Jobs (class specializations) and encourages it. While your initial selection should be prioritized, you will most likely find yourself overleveled for the main quests and at that point, there is no harm in switching over to a new Job. Not only will it help mix up the gameplay, but if a Job is a lower level than your highest level job, it will accrue XP at a faster rate. It’s a win-win!
One thing to keep in mind with Class selection, however, is that your choice will determine which city you spawn in at first. That can make things tricky if you’re trying to start from scratch with a friend. Still, you should go with the Class that calls to you the most (we’ll discuss what to do if you’re in a different starting city as your friend a little later).