Create a shape on a shape layer

Select a shape tool or a Pen tool. Make sure that the Shape Layers button is selected in the options bar.
To choose the color of the shape, click the color swatch in the options bar, and then choose a color from the Color Picker.
(Optional) Set tool options in the options bar. Click the inverted arrow next to the shape buttons to view additional options for each tool. (See Shape tool options.)
(Optional) To apply a style to the shape, select a preset style from the Style pop-up menu in the options bar. (See Apply preset styles.)
Drag in your image to draw a shape:
- To constrain a rectangle or rounded rectangle to a square, to constrain an ellipse to a circle, or to constrain the line angle to a multiple of 45 degrees, hold down Shift.
- To draw from the center out, position the pointer where you want the center of the shape to be, press Alt (Windows) or Option (Mac OS), and then drag diagonally to any corner or edge until the shape is the desired size.
  Drawing from the corner (left) and drawing from the center (right)
  
  Note: Drawing from the center out is the default behavior of the Star tool in Illustrator and the Polygon tool in Illustrator and Photoshop.