A circle is where many, but not all, witches do their magical workings and where they worship their Deites. There are lots of different ways to cast a circle.
How you cast your circle is pretty much up to you. Here are some examples if you have never cast a circle before to give you some ideas.
How to Cast a Circle by Arwen Nightstar
Circle Casting (an outline) by Laurie Cabot
Casting a Celtic Circle by D.J. Conway
Circle Conjuration by Doreen Valiente
Casting a Circle for God by Silver RavenWolf
Casting Circle in Your Hand by Silver Ravenwolf