Global Wars & Enable PvP events are settings in your guild menu - the Global Wars I believe your guild also has to declare.
For the events themselves, they happen every hour, the Town Struggle there is a green moongate by banks you enter 15 min before the event, the Corpse Creek, Dungeon Flashpoint and Castle Siege events you just need to go into the area where they are being held, to get triggered into the event, then it's about holding various points, and collecting points while PvP'n, and trying not to lose points to the other combatants by dying to them... Mostly they are group events, with the exception of Town Struggles, while it's one team vs. the other, it's balanced by assigning players to teams, so you can join individually.