Use the Ellipse Tools to draw precise circular and oval shapes. Clicking the small arrow next to the tool's icon will display a menu containing the sub-tools below:
The thickness of the ellipse's border is controlled by the Line Width slider on the Option Panel. The Fill Selector and Color Palette can be used to change the border/line color, interior fill color and the fill method (solid color, gradient, image tile, pattern, etc.).
Ellipses are normally drawn from a corner and dragged to the opposite corner to produce a shape of the desired size. Instead, if desired, you can select the Option Panel's Draw from Center option to start in the center and drag to an outside edge to create these shapes.
Holding down the Shift key while drawing will restrict ellipse objects to perfect circles.