How does the Party gain control over the people in 1984?

The Party gains control over people through constant surveillance, propaganda and thought control. 

In Oceania, Party members live with viewscreens that can never be shut off. These screens not only show incessant propaganda about how life is ever improving (it is not), the screens can be used to spy on people. For example, when Winston does not do his exercises properly, the voice of a person watching from behind the screen scolds him. Neighbors are...

The Party gains control over people through constant surveillance, propaganda and thought control. 

In Oceania, Party members live with viewscreens that can never be shut off. These screens not only show incessant propaganda about how life is ever improving (it is not), the screens can be used to spy on people. For example, when Winston does not do his exercises properly, the voice of a person watching from behind the screen scolds him. Neighbors are also encouraged to spy on each other, and children are expected to spy on their parents and report any deviances. In fact, Parsons is arrested because his children denounce him. People are clearly afraid to challenge the state in any way because of this constant monitoring.

Second, propaganda is almost ceaseless. The government owns all the airwaves, newspapers and publishing and uses these media to constantly whip up fear about enemies and send out whatever message it wants about the Party or Big Brother. Posters everywhere remind people that Big Brother is watching them. The daily two-minute hates direct people's repressed sexual energy toward whatever target the government wants to attack.

Third, the Party demands not just outward conformity to its rules, but inward conformity. People are not allowed to think thoughts the Party does not allow, and the Party works to reduce the number of words in the language to a bare minimum so that people can't formulate subversive thoughts. This extreme emphasis on inner conformity leads to self-censorship as people fight to avoid arrest by the ever-lurking but mysterious Thought Police.