You must be in a corp that does FW. If your current Player owned Corp doesn’t do FW, you must leave it.
You can be in an NPC Corp that does FW, you can select it as part of the sign up process
You don’t have to join a fleet, you can do FW as a Solo player
Most of the FW content is only in specific low-sec systems. With that said, anyone who’s part of the enemy faction can attack you anywhere in New Eden (CONCORD won’t interfere)
As long as you want
You can leave FW anytime (you probably have to be in a station to do it)
you leave the player corp you are in then enlist with a faction and it will put you in the npc corp of that militia.
I do strongly recommend joining a player corp though if you are new. if you are looking to join caldari or don’t care the main corp in my alliance is focused on players new to fw. if you want to join another faction you can normally find a corp once you join in the npc corp and just ask around in the militia chat.
the cal/gal WZ is normally more active but amarr and minmatar are still fine to join. no matter who you go with you can always feel free to mail me in game for any questions I love helping new players in FW.