As previously stated, you don't necessarily need a circle. However, if you feel that it would help you, you could cast one.
Also, if you have ever read "Wicca" by Scott Cunningham, he mentions something called "cutting a doorway" which, more or less, is creating an opening in the energy of your circle to allow you to pass though it without passing through the circle itself.
You create the opening through visualization and control of energy flow. Visualize the energy flowing into your athame or hand (gently pull it back into either the athame or yourself) to create the opening and then visualize it returning back into the circle to reseal it (gently force the energy back into the circle). Also, the opening only needs to be big enough to allow you to enter and exit the circle, so you don't need it to be a huge hole in the circle. Hope this helps, or at least gives you information to use in the future.