Just to add to my answer above: the difference between what Julia and Winston want is that Julia wants primarily to satisfy her own instincts and desires whereas Winston's main aim is to try and change society. They both rebel against the Party for quite different reasons, therefore.
I'll give a few suggestions for what you could include in this essay.
For the introduction, you could first of all briefly outline the background to the rebellion of these two characters: they rebel against a totalitarian state which aims to control every aspect of their lives and to brainwash them into blind acceptance of the ruling, all-powerful Party. It is this brainwashing that they strike out against, while projecting an outward appearance of conformity as they go about their daily routines as Party members. They are both very courageous as they know that ultimately their thoughts and actions will lead to their capture, torture, and death. However, they are very different in practically every other respect: in terms of age, experience, outlook and temperament. These differences in character influences the nature of their revolt; Julia's is physical, sensual, while Winston's is intellectual.
Having summarized the background and the characters in your introduction, you can now look at their differences in more detail. You could begin with Winston, as he is the central character of the book. You could briefly describe his job and the general nature of his day-today existence which appears dull and stifling and overall very oppressive. The important point to note, of course, is that he is one Party member who has retained the sharpness of his mind as is clear from his secret journal where he continually jots down his philosophical musings. He thinks deeply all the time, tries to do his own private research as opposed to the official Party lines that he is obliged to spout in public, is constantly on the lookout for other people he thinks may be secret rebels. (Ironically, he is not aware at first that Julia is a rebel, while mistakenly believing that O’Brien is.)
Julia, on the other hand, appears in a more physical light from the first. She is young and pretty, whereas Winston is almost middle-aged and feels himself to be distinctly unattractive. While Winston is reserved in manner, she is more forward; she is not afraid to actively seek him out as a fellow-rebel. She is always aware of the more physical aspects of their affair whereas Winston tends to think of it in a more abstract way, as 'a political act' against the Party which strictly forbids sex for pleasure. Similarly, she is aware of current events only as they affect her materially: increase or decrease in rationed luxuries like tea and sugar and chocolate, and so on. She also likes material comforts such as make-up which are officially denied to female Party members. She revels in sex, in physical delights, and although she is a loyal, stalwart companion to Winston, she does not share his interest in intellectual and political discussion; for instance, she falls asleep when he is reading Emmanuel Goldstein’s book to her. There is more than a grain of truth in what Winston says to her rather jokingly at one point: ‘you’re only a rebel from the waist downwards’ (chapter 13).
While Winston constantly broods on the inevitability of their eventual capture and death, Julia tends not to think that far. Instead, she lives for the moment:
'We are the dead,' he said.
'We're not dead yet,' said Julia prosaically.
'Not physically. Six months, a year -- five years, conceivably. I am afraid of death. You are young, so presumably you're more afraid of it than I am. Obviously we shall put it off as long as we can. But it makes very little difference. So long as human beings stay human, death and life are the same thing.'
'Oh, rubbish! Which would you sooner sleep with, me or a skeleton? Don't you enjoy being alive? Don't you like feeling: This is me, this is my hand, this is my leg, I'm real, I'm solid, I'm alive! Don't you like this?'
She twisted herself round and pressed her bosom against him. He could feel her breasts, ripe yet firm, through her overalls. Her body seemed to be pouring some of its youth and vigour into his.
'Yes, I like that,' he said.
'Then stop talking about dying. (chapter 11)
In a word, Julia is practical, while Winston is intellectual. She rejoices in her own individuality, her physical needs and desires. Winston, on the other hand, as is clear from this excerpt, is always thinking in terms of the human race, of the future, rather than being solely concerned with his own individual needs and desires.
To conclude your essay, you could maybe make the observation that, although so very different, Winston and Julia make a very good team; they complement each other well, and right up to the moment of their capture, they show unstinting love and loyalty for each other.