Janice - great question!
Here is how I started...
First I used scratch paper for practice and I started with monochrome rather than multiple colors until I got what I was looking for and expanded from there.
Just play with layering the images, using all styles of images as well.
It doesn't necessarily have to cover the whole piece of paper either.
Here is an example of a card I did using the Charming set.
To me collage making is just layering images. You can add sponging to add color as well.