Fantasy Online FAQ
That should answer most questions about the game.
As for the specific ones in this thread:
How to join a guild: Near the market in Crab Coast there is a man behind a desk who will show you all the open guilds you can join. You can also be invited to a guild by a high ranking member of that guild.
/guildleave will make you leave your guild and return your guild donation to you.
Is this Final Fantasy Online? Not in the slightest. As far as I know, there is absolutely zero relation between the two games.
If you are having computer problems:
Try Clearing your cache
Close any other windows you have running
Update flash player
Try with another browser
Try playing on the main site
If those don't work then you must have a computer older than I am.
I highly recommend you visit the official Fantasy online forums if you have any more questions. There's loads of info there and it's fairly active.