In 1984, how does Winston react to the note from Julia before reading it and after reading it?
Your question is a little vague because Winston couldn't react to the note before he read it :) I think you are really asking about how the note changed him. Before he read the note, Winston didn't have a relational life. He was a wreck, lived in a hovel (by our standards, at least), did his mind numbing work, drank his Victory Gin, and started over the next day.
After he read the note, he had something/one to live for. He begins arranging "accidental" meetings with Julia, planning his life around her and their meetings in the upstairs room. Of course, all of this is done under the watchful eye of the Inner Party and will come to naught, but for a brief time, Winston has someone he cares about and it totally changes his life.
Before the note, Winston was scared that Julia (dark haired girl) was a part of the Thought Police and that this could be a note saying that they caught him and were going to vaporize him. But, after reading the note, Winston was relieved, surprised and excited because he realized he wasn't in trouble, but that the girl he had been admiring for so long rebelled against the government and loved him.