I use a free programme called Gimp but will say the process would be the same for most if not all layer based graphic editing programmes.
Open all the images you require or wish to make a collage of. On a main image, the one you are going to layer other images on create a layer. Note I do not know if photoshop allows you to paste as a layer. Copy one of the other open images and paste this onto the main image. Now make sure you paste either as a layer or paste onto the layer you made. The reason for the layers is the ability to move each/resize each image.
Clear as mud?