There are many definitions of critical thinking. Read a few. Memorize at least one that makes sense to you.
Evaluate and analyze.
Thinking about thinking to improve thinking.
Recognize assumptions, evaluate arguments, draw conclusions.
"Critical thinking is the intellectually disciplined process of actively and skillfully conceptualizing, applying, analyzing, synthesizing, and/or evaluating information gathered from, or generated by, observation, experience, reflection, reasoning, or communication, as a guide to belief and action." - criticalthinking.org
Going through the Elder Paul model's Elements of Thought wheel
TED-Ed 5 tips to improve your critical thinking (4:15)