Have you ever started highlighting a text and then realised it is nearly all highlighted? If you use a highlighter, it can be helpful to make a note in the margin of why that particular section of the text struck you as important.
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.
Remember to make a note of all the information you will need in order to cite the text in your essay.
You might wish to keep a reading log.
This video has some useful tips for taking notes.