Simple photo border
This tutorial will show you how to create a simple border around a photograph for display on a computer monitor. I'm going to use an image that's 532x800 pixels.
Step 1
Open your photo and resize it so the long side is 800 pixels.
Your layers screen will look like this:

Double click the background layer and just click OK in the window that opens, that will make the layer floating.
Step 2
Create a new layer (Layer->new->layer) and call it 'Border'. Then drag this layer behind the original photo. This will be the border colour.
Create another new layer and call it 'Stroke'. Place this layer at the top of the stack if it isn't already at the top.
Step 3
Now we're going to create the room for the border. I will create a 40 pixel border allround. Then add another 40 pixels to the bottom to make extra room for text.
If you want to create a border around a full size image, you'll need to use larger numbers. Play with the extra size untill you're happy with it.
First select the canvas size dialog (Image->Canvas size). Make sure the 'Relative' checkbox is checked.

The extra 40 pixels at the bottom are done basically the same with the following settings:
Step 4
Now it's time to add the colour to the border. First select the bottom layer (Border) and fill it using the bucket tool (or use the keyboard shortcut alt+backspace). I'll use black for this example.

Now, to separate the picture from the black border, select the top layer (Stroke). Next press and hold the ctrl key and click on the layer with the photo (click on the thumbnail).
This will create a selection that's exactly the size of your photo. Now make sure the Stroke layer is still the active layer. Then from the menu select Edit->Stroke. And use the following settings:

This is the result:
Step 5
Add text. This is pretty much straightforward.