Well this a small tutorial to do basic borders.
Step 1. Open your image.
I have used the following image:

Step 2. Create another layer.
Layer ---> New ---> Layer
Step 3. Fill the layer with White.
Edit ---> Fill ---> White
Step 4. Change the blending mode of the filled layer to 'Darken'
Step 5. Edit ---> Stroke
change the width to 3px and select the color black
Step 6. Again do another stroke and change the width to 2px the color is your choice (Best if it is almost the same as your image color)
Step 7. One more stroke, this time changing the width to 1px and the color back to black. and your image will have a border like mine:

Thats how to make a simple border. :)