make a new layer, then go to your layers palete (the box on the bottom right that shows each layer ) right click on the one that shows your new layer and select "blending options". then go down to where it says "stroke" . check the box and double click that section and then set the properties. make sure its set to inside
and just play with the size and color until you like it.
if this is confusing just let me know, ill help ya get through it.