Create a new Document in Photoshop this can be any size you choose now select a photo you might want to use and now add the image to the document you created 2 seconds ago.
Select the Marquee Tool you can do this many ways by clicking on it or by pressing Ctrl + M this will select the rectangular Marquee tool but we don't want this one we would like to use the Elliptical Marquee tool.
If your photo is of a person you might want to hide or blur out a special area am going to choose to use the face this is a example such as when you see them on tv and such when you cant see there face. now with the Elliptical Marquee tool create a circular around the man's or women's face and create a new layer after doing this you will need to go to Filter - Pixelate - Mosaic and play around with some of the settings.
Mosaic Setting you can play around with the settings :
