Now that you have your
yarn and hook, you're ready to start.
The first thing you need to do to make anything in crochet is to make a slip knot. (There are always exceptions.)
These instructions are for right-handed crocheters. If you are left-handed, just switch the directions and hold a mirror up to the side of your screen ^_^
Find a yarn end (preferably the one on the inside, again, more on that later) and lay the yarn out with the end of the yarn to the left:

Lift the yarn up to the right of the end and fold it over itself making a loop:

With your thumb and pointer finger, reach through that loop and grab the yarn to the right of the loop (towards the ball) and pull it through the loop:

Pull on the yarn hanging down from the loop (I needed one hand to take this picture, so imagine my other hand holding the loop at the top) :

Grab your hook and insert it in the loop you made:

and pull on the yarn leading towards the ball (known as the “working yarn”) and the loop will tighten around your hook. If it doesn’t, try pulling on the yarn tail. If that works, then you did something differently than the instructions. Take your hook out of the loop and keep pulling on the yarn tail until the slip knot comes out, and try it again. It’s important that the working yarn be the side that closes the loop.