Choose Tools > Style (from the Tools menu at the top of your screen).
Press the S key on your keyboard.
In the Tool Options pane, click Add Style and choose Stroke. You can add as many of the same styles as you like.
Customize the Stroke layer style:
Stroke Width: Drag the Stroke Width slider to adjust the width of the outline. Hold down the Option ⌥ key to extend the range of the Stroke Width slider. You can also enter a value from 1 px to 200 px.
Stroke Position: Click the Position pop-up menu below the Stroke Width slider to choose the position of the outline. Inside makes the outline expand into your selected layer, Center makes it expand both inwards and outwards from the edges of the layer, and Outside makes the outline appear entirely outside your selected layer.
Stroke Style: Click the Stroke Style pop-up menu to the right of the stroke position menu and choose a solid, dashed, or dotted outline.
Stroke Fill: Click the Fill pop-up menu below the Stroke Style pop-up menu and choose either a Color stroke for a solid color outline or Gradient to fill the outline with a transition between multiple colors.
Stroke Color: Click the color well to choose a color or click to pick a color from the image.
If you have applied multiple Stroke styles to a single layer and you’d like to rearrange them, simply drag the style up or down the list of styles.
To reset all styles applied to an image, click Reset at the bottom of the Tool Options pane.