Hiya Kim.
Make sure that 'Feather' is set to zero in the Option palette for the Marquee tool. This is where you are getting the fuzziness from.
Select All, then hold down the Option key. This allows you to subtract from the original selection. Click and drag inside the original selection to leave the thickness of the border you require. Edit > Fill, or use the Bucket tool.
You can also inset the selection from the edge of the bookmark and 'Stroke' a line using the foreground colour.
There are loads of variations on this, but start off with what you are doing and then build up your knowledge as you gain experience.
If / when

you need more info, just ask.
Al