Circles

Click and hold the Rectangle shape toolbutton to see more shape options...

To draw circles or ovals, you need to select the Ellipse tool. You'll notice an ellipse/circle has 4 points.

To draw a circle, you'll need to hold SHIFT while you draw it. 

When you select a point from a curved shape, you will notice blue handles. You can adjust these. When you adjust one, it adjusts the other one, like a see-saw.

If you want to adjust just one side/curve, hold ALT while adjusting one handle.