Pretty much all you really have to worry about is doing the Main Story Quests, they'll be marked with a specific quest marker. Any quest markers you see that are just a generic exclamation mark are side quests, some of them can have a bit of interesting side story but are 100% skip able. The markers with a plus in the bottom right hand corner indicate content unlocks. As for what to start out as, Lancer/Dragoon is fine, most of the base ARR jobs play well enough, but don't feel like you are locked in to one thing, you can always unlock the others later. Regardless of DC and server expect lots of the usual LGBTWTFBBQ+ shit in every major hub and for it to be infesting Party Finder once you unlock that too. If you're playing with friends that will help a lot as they can probably help you come to grips with some of FFXIV's many obtuse mechanics and systems. The main downsides to free trial is you can't create parties, only join ones that invite you, you can't sell on the Market Board, you can't trade with other players, and a few other restrictions, but you currently have access to all content to the end of Shadowbringers, so 80 levels of content.