A the bottom of your layers palette there is a row of icons. One of them is a little "fx". Click on that and choose "Stroke" from the pop-up window that appears. You'll get the Layer Style window, with Stroke already checked and active. The stroke is an outline around any brush strokes in the layer you had selected when you activated the layer styles. By default it should be a 3 pixel black outline, but you can adjust the parameters in that window. Once you click OK, you'll get a couple of new lines in your layers list: Effects and Stroke. If you double-click the Stroke line, it will call up the Layer Styles window again, letting you change the parameters even after the effect has been applied.

The effects will also be added to new brush strokes you place on that layer, and if you erase something, the effects will go away. You might also like to play with the glow and bevel effects.