If you want a filled out circle just use the select pen, adjust its size to how big you want your circle and tap it once on the canvas without moving your pen anywhere. You now should have a perfect circle selection. Then fill the selection with any colour you want. If you want an outlined circle you can do the same but afterwards make a slightly smaller selection inside your circle then erase anything that's inside. You can still resize the circle later if you want it to have another size. Hope that helps.
use google docs. if you use gmail then use your gmail account to log in. open a new drawing, create a circle, save as png. you will have a circle on a transparent background. Thats what I do with my shapes.