To only open an image just: file, open, find the photo you want, and double click.
For open an image into another image: opem both of them on photoshop.. and then drag the picture you want to the picture you want to the other image (if that makes sense).. OR open the first image and then open the other image too.. then tranform and scale it so you can move it around and change the size.

If you are "Amateur" on Photoshop, and you have a PC:
Just open PS and double click the main gray area. A Windows explorer window will appear, then just find your image and click OK. For those using Mac, go to:
File > Open > Find your image on Finder.
Then you repeat all the sequence importing all different images you want. There you are. Now you may like to mix them by dragging any of the images over the other. Just have fun!