1. First, open a new doccument in Photoshop by clicking in the File> New.
A window will appear and change thesettings as following image.
2.From the tools box select theoval selection tool as shown in the following image:
3.Holding down the shift key draw acircle [holding down shift key insure that the circle is perfectlysymmetrical].
4.Select the paint bucket tool fromthe tools box and fill the circle with your desired color as shown below: