how to create a canvas collage with multiple pictures

There is no automatic way to do this with Photoshop. Follow Nancy OShea​'s directions to manually build your collage. I would start with a new document 10"x10" at 200 ppi.

1. Create your canvas.
2. Open an image and hit Ctrl+A to select All.  Hit Ctrl+C to copy. 
3. Switch to your canvas and hit Ctrl+V to paste.
4. Repeat with other images.

Ideally you want to keep each image on it's own layer as shown in my screenshot below.  That will give the freedom to resize and transform images separately.

Nancy