This is the method that I use for making a slip knot and it’s the first thing that a beginner should learn as it’s often the first step in many crochet projects.
You want to start with a tail of about 6 inches. Take your yarn and wrap it away from yourself around the index finger of your left hand twice. Hold your yarn tail between your thumb and middle finger.
Take the loop furthest from your fingertip and pull it over the second loop while still keeping it on your finger. Then take the second loop and draw it over the first loop and off your finger. Pull the first loop tight while holding onto the yarn tail.
You’ve made a slip knot! Insert your hook into the loop and pull on your working tail (the strand of yarn coming from the ball) to tighten.