Like any part of digiscrapping (& most specifically within PS) there are several ways that you can do most things - just by trial and error you will find the way that works best for you.
#1 Way - You can use the Shape Tool - if you go to it and then to your top menu you will find that it has a fly out menu with an unbelievable number of choices.
#2 Way - I tend to use the rectangular marquee tool for rectangles and squares and then once I have "drawn" the size that I want I fill it with color or pattern with the bucket tool.
#3 Way - If you already have the photo in place - then you can (make sure that your layer with the photo is active) "Control>click" which will place the marching ants around your photo. Then go to Select>Modify>expand (& enter how much larger you'd like the rectangle to be).
Then create a new layer (leaving the rectangle marching ants still going). You will see that you have a completely new layer (with the marching ants rectangle) and you can now fill that rectangle with color of your choice.
#4 Way - Make sure your photo layer is the active layer you are working on and then go to Edit>Stroke (and enter value of size here) and then color choice - this will add a border directly around your photo.
HTH
Trace
