1984 is what we call a "dystopian" novel. A dystopian novel depicts a corrupted or evil future, unlike a "utopian" novel which depicts a perfect future.
In 1984, the future is controlled by the leader of the Party, Big Brother. Big Brother's goal is to bring everyone under the Party's control, including their thoughts and emotional attachments. They do this by conducting constant surveillance and repetitive brainwashing techniques.
If I were going to warn Winston Smith about Big Brother, I would make sure to tell him not to trust people who seem to be on his side. Winston is ultimately betrayed by a man named O'Brien, who he thinks is a fellow dissenter. O'Brien ends up being the man who interrogates Winston in the Ministry of Love. It is there that Winston learns that it isn't the rule-breaking that Big Brother cares about, but rather the fact that some people have original thoughts that do not support Party goals, and that they have strong emotions for someone other than the Party.
He is also betrayed by a disguised member of the thought police, Mr. Charrington. So my letter might specifically urge Winston not to trust him.
The Party's power is described as being so pervasive that there isn't much hope of overcoming it and still avoiding detection. Winston's relationship with Julia was meaningful to him, but it led to his ultimate capture and torture. It would be hard to advise someone to have any meaningful emotional relationships, but in this dystopian world it would be the only way to be safe.