The first step to crocheting in the round is to create a ring. This can be done with a foundation ring (ch4 and join with a slip stitch), or by creating a magic ring (also known as magic circle or adjustable ring).
An advantage of using a magic ring is that it can be pulled tight which closes the centre of your circle, especially useful for hats and amigurumi.
Take a look some of my patterns:
I use UK terminology, click here for a handy conversion chart to US terms