Well for the circle it s quite easy When you load photoshop you should see your toolbar on the left the icon your looking for is something that look like a square.

Left click on it and hold you will see a circle select it then you can doing your circle where you want by doing left click and dragging till good size after that you put the color but you should try to learn the basic of the program before doing the planet some experiencing wouldnt hurt dont try to make anything just know what thing does.