As a reader, you are not a passive participant, but an active and inquisitive one. A “critical” attitude will make anything you read richer and more useful to you in your classes and your life.
To read critically is to make judgements about how a text is argued. This is a highly reflective skill requiring you to “stand back” and gain some distance from the text you are reading.
You might have to read a text through once to get a basic grasp of content before you launch into an intensive critical reading.