Not wanting to sound negative, but that sounds like a vague assignment.
When I was at school we had to read many important books as part of our English Literature classes - but all our home assignments were given a specific brief - discuss the characters, the writing style, the similarity between book 'x' and book 'y'.
General discussions were carried out in the classroom.
Given the nature of your assignment I would say that you should consider the following questions when looking for text to highlight:
(1) Does this describe the character? Does this show how the character is thinking? Does this show that a character has changed his mind from his position at the start of the story? If you have an opinion on a character, does this bit of text confirm or deny your opinion.
Most of the discussions we entered into in classes were based around the characters of a novel and how evolved throughout the story, how they interacted with each other and our own personal opinions of a character (which we then had to back up with examples from the text).
(2) Is this a good turn of phrase? Does this sentence use a particularly good simile, metaphor, oxymoron? Is it humourous? Is it an example of irony or satire?
Conversely, do you find the sentence hard to understand? Is it perhaps badly written or dated making it hard for a modern reader to understand the author?
Other things to look for in writing style is does the style change from chapter to chapter? Are some chapters or passages told from one point of view, while others are told from another point of view. Does the tense of the novel change? Are some passages told in first person while others are told in third person?
(3) If you have seen a film of the book, don't be afraid to note any differences between the book and the film. It shows you have read the book and also that you have seen the film. If there are differences, are they important?
I know that they're a lot in there, and it sounds like you'll be writting a page of notes for every other page of the book. But remember what's most important is what's important to you. Your opinion of the characters, story and writing style is formed while you read it, so note anything that supports or changes your opinion - note it down.
Being as opinions are personal there is no right or wrong answer (one of the reasons I loved English Lit so much at school)
as long as you can support your opinons and theories with text from the book
I must say, without a specific brief it does sound like a hard assignment
), and underline important text, make notes, ect. Now, I have never annotated anything, so I was wondering if you all could tell me how I can distinguish something important or someting not. This is my first assignment, and I want to make a great impression. 
). I know it's vague, but sometimes that's the way it goes. You just sort of make it up as you go along. If you notice something that the teacher might be looking for in the assignment, highlight it. If you see something that you yourself like and can relate to (like a good quote that stands out) highlight it. Highlight all (or most if there's much of the same kind) the literary devices you see used, and key plot points that you think will influence the rest of the book. Just play it by ear and think small, and you should be all right. 
