The day of Harry's 18th birthday, he'd been congratulated by the Dursleys with time. Yes, time. They took their oh, so ungracious time and used it to pack his bags. He found them lying on the front porch. Harry had expected this so he'd already arranged for himself to move into his own secret house, one big enough to assist members of The Order when they needed a place to hideout or assemble. On the 7th of June, Harry received a letter telling him that his uncle Vernon had died due to choking on a ham bone. He, of course, did not go to the funeral because he'd, naturally, be blamed for his uncle's untimely death.
With the addition of Harry, Ron, Hermione, Fred, George, Neville, many other Hogwarts alumni (mostly people from the D.A.), and estranged house elves, The Order has grown to twice its size before Voldemort's official comeback. The Order has placed its headquarters at Harry's place because Grimmauld Place was to dangerous a house to stay, considering Kreachers suspicious behaviour such as disappearing for large chunks of time and refusing to leave, even after Sirius' death.
Harry has gone into hiding because higher members of The Order feel it is not wise for Harry to battle or encounter Lord Voldemort just yet. After Hogwarts, Malfoy, Goyle, & Crabbe went missing, their whereabouts unknown, but everyone has a pretty good idea who they're with. Fudge, like other Ministry workers, was sought out and brutally killed by a band of Death Eaters; Percy was destined for the same fate but ultimately just tortured into madness, very much like the Longbottoms. While the wizarding world is falling into darkness, Hogwarts remains one of the only safe places left, mainly because Dumbledore and the Order heavily protect it.
The Order has largely grown but Voldemort and his Death Eaters have continued to thrive as well. Many wizards and witches in very high places have come out of hiding and now openly express their support for Voldemort. Azkaban prisoners, the dementors, and many evil creatures have undoubtedly chosen a side. As the battle for the Wizarding & Muggle world rages on, many lives, both good and evil, have been lost for their cause. With more and more danger approaching each day, Harry must find a way to end this war once and for all.