Developed? If there on a CD they are digital images and don't need developing. Just pick the ones you want and have them printed out. And you still get to print others later. If the photographer and the images he took were good you got a GREAT deal.
Make a new blank canvas in PS. Set the dimensions and resolution.
Make sure the dimension are big enough to hold both of the photos you want to join.
Then open up the photos you want together. I'd put them on their own layer. So if you have 2 photos, you'll have 3 layers.. the main...
There are TONS of photography help on the net. Join some forums.
Take PLENTY of pictures, and critique them ... what you liked, and what you didnt like about them. Oh .. and learn your camera. Its your tool