Welcome!
That's a very hard question to answer sadly - as each FF game in the series is a unique setting/world and storyline (worlds with more Western-themed 'sword and sorcery' type fantasy setting were common for most of the earlier games, albeit often with a slight sci-fi twist, but more explicitly science-fantasy/steampunk and science-fiction trappings were often associated with the later titles starting with FFVI), the series is more defined as common reoccurring ideas, elements and concepts that appear throughout each individual FF game and title (like items, magic, monsters, character classes, chocobos and moogles, a character named Cid, a dimension-hopping antagonist-cum-possible-ally named Gilgamesh, protagonists known as the Warriors of Light, elemental Crystals etc).
As a general rule though, most FF games usually feature the player taking the role of an group of (usually young) individuals who are thrown together by fate to save their world from an antagonistic force that would either destroy or subjugate it, but often the actual antagonist itself isn't actually visible or known, and may be waiting in the wings while another lesser evil is the one out in the open causing trouble, often resulting in a 'bait-and-switch' type occurrence near the end of the game where the visible villain is unceremoniously disposed of/defeated and the 'real' villain makes him or herself known. 'Saving the world from an apocalypse' is a very common end game theme that appears in most FF games.
Really the best FF game to start with at this point would probably be FFVII, as that was the 'breakout hit' for the series and to this day is the one credited to introducing JRPGs to a wider audience. If you're fine with early Playstation One visuals and sound (the game uses pre-rendered 2-5D backgrounds and somewhat superdeformed 3-D character models, except in battle scenes) then this game would be perfect to start with. :)
I'm sorry if that description was a little complex - studying game design kind of does that. :p If you're looking for more information about the series period, try here (
https://en.wikipedia.org/wiki/Final_Fantasy) for a general rundown and the FF Wiki (
http://finalfantasy.wikia.com/wiki/Final_Fantasy_Wiki) for pretty much everything you would want to know about the series and the many tie-ins, prequels, sequels and spinoffs released with it. :)
Anyway, welcome once again to the Shrine! :D