Ordinarily, if your instructor asks you to write a summary of something you've read, it is to help you to clarify what you read and to enable the instructor to determine whether you've understood it. Because this kind of assignment is limited to presenting others' ideas, an instructor will often combine it with a reaction assignment, to find out what your opinion is. In a reaction paper, usually after you have stated the author's main ideas and main supporting evidence, you state your own responses to those ideas, backing them up with your own evidence and thinking. In many cases your instructor is expecting a particular kind of reaction, for example, a statement of whether you agree or disagree with the text and your reasons. Find out the specific expectations.
The following is a reading-writing process that works for many students when summarizing thesis-support articles. You can adapt it for longer and different kinds of texts and to your own process, with guidance from your professor.