Technically speaking, Winston is an insider, at least to the extent that many people in his society are. He isn't one of the proles, and works in the Ministry of Truth. So, he is technically part of the infrastructure of the government, which makes him an insider of sorts. He has started to rebel, in his mind and heart, long before the novel even starts. Even from the opening pages, we see that Winston is unhappy, questioning of the government and the way that life is, and searching for someone like-minded to connect with. At his apartment, he has been writing down his persona, and rather rebellious thoughts for quite some time in a journal of sorts, out of view of the cameras. So, his rebellion began long before we meet him at the beginning of the novel.
His first overt act of rebellion, however, comes in his relationship with Julia. Before this point, he has only looked and people and wondered about their status, and whether they feel the same. He never has the courage to act on it. Julia, when she initiates, gives Winston hope that someone else is out there who feels like he does. Going to meet her is his first act of open rebellion. Before that, he had done little things like buy the journal, visited the prole section of the city, etc. But meeting Julia is huge, and potentially life-threatening if discovered.
As a person who works inside of the Ministry of Truth, Winston has a distinct advantage in knowing, first-hand, that his government lies and changes history in order to maintain its power. He knows this, and that information works to his advantage in the fact that it is something that he has over the government, but it is to his disadvantage because it irks him, and makes him unhappy and discontented to know that is being controlled by a bunch of liars. So, it works both ways for him. Because he does work where he does, it is a disadvantage because he is more closely watched. The proles, for example, are mostly left alone, but Winston, because he is privy to the government's secrets, is watched like a hawk for any signs of rebellion or treachery. That is a huge disadvantage, and one that keeps him from being happy.
He is an outsider to the inner circle of the Party, and that is a disadvantage because he does not get the nice things, does not have the privileges that those members do, and does not have the truth behind their plans or the power to gain his freedom and happiness. It is a disadvantage becuase he never knows that he is being watched, and so acts in ways that leads to his torture. Being an outsider to the inner party is an advantage in the fact that he does not know their darker secrets and so can't be held accountable for them.
I hope all of those thought helped; good luck!