Unfortunately, you have posted to the Premiere Elements Forum. You want the Photoshop Elements Forum.
They will give you the steps, but there are many ways to create a photo-collage. Those can differ a bit, but basically you will Open a background Image (can be a New Image w/ a colored background or an existing Image), and then the ones that you wish to include. Click on the Background Layer of each of those, and drag to the background Image. The next step is for Photoshop, and might not work in PSElements, but hit Ctrl+T (Free Transform), hold down Shift to constrain the Aspect Ratio, click one corner and drag that, to decrease/increase the size. Repeat.