You will become a newt. Kidding.
There are a couple reasons why you cast a circle, it's considered 'holy ground' or a place between worlds where you can speak with the divine [think of it like church but no building]
In spell casting it effects the energy around you, some feel it makes casting easier, but primarily it's a barrier to protect you from any negative energy/spirit who might attach itself to you while you cast.
To cast a circle there are many methods, look through the articles and find one that works for you. Basically you walk clockwise stopping in each of the four directions to call on the element [some use watchtowers, others use angels, I call on dragons, whatever works for you] when you are ready to leave, walk in a counterclockwise direction thanking the elements. There is a lot more to it, but basically that's it. Be sure to charge and visualize the energy forming the circle. To know you did it correctly you should feel it, the energy is different, you might feel warmer, or can sense where the circle ends.