Highlighting a few sentences and forgetting about them until you start to write an essay is not enough. If you use a highlighter, always make some notes in the margin:
Revisit your highlights from time to time and reflect on how you might use the information.
In addition to highlighting, summarise or paraphrase the text in your own words.
As well as making notes about what the author says make a note of your reactions.Note any questions you have.
Make a note of all the information you will need in order to cite the text in your essay.
Try to keep a reading log.
Watch this video on note taking. The first part refers to taking notes in lectures and the second to taking notes when you are reading.