When looking for a focus to work with on a collage, I want something to stimulate me, something to use as a foundation for the image I want to produce.
Sometimes I use a photograph, other times I use my art that I have previously photographed. Each section must have an obvious focal point that will draw the viewers` eye into the picture. This will act as a stimulus and I can then work on creating fluency throughout the composition.
A face is an obvious focal point.
But other subjects can be just as effective when designing collages.