The Oval tool is used to make circular objects. Simply select the tool and drag on the Stage to draw a circle that spans between the start and end points.
SHIFT key : Round Circles
You can press the Shift key while dragging to ensure your circle is perfectly round.
You can specify the colors with the Stroke and Fill Color Tools.
Ctrl+G : Grouping Outline with Fill
When you draw a circle, Flash actually creates two objects: the fill and the outline. This may cause problems if you try to move your circle to a new position, because if you fail to select both objects, only the one selected will be moved. To avoid this it's often a good idea to "group" the outline with the fill, thus locking them together as if it was a single object.
1: Make sure both the fill and the outline is selected.
(You can do this in two ways: 1) Select the Arrow tool in the toolbox, and while holding down the SHIFT key click on both the fill and the outline. 2) Select the Arrow tool in the toolbox, and drag a rectangle outside your circle)
2: Press Ctrl+G to group the selected objects.
(If at a later point you want to "break" apart the grouped objects, you can do it by selecting the grouped object then pressing Ctrl+B).
Precise size and positioning
You can position the oval perfectly using the info panel.
Info Panel : Window > Panels > Info
The "square" connected to the line where it says "Click this square" is used to set the origin for coordinates - either at top left or center of the stage.
Fields for X and Y are used to enter the position of the selected object.
Fields for W and H are used to enter the width and height of the selected object.