Your question is a bit vague, but have you looked into Google Picasa? It has a function that allows you to pick picture and automatically put them into a collage. Is is pretty basic, but a pretty good starting point.

Your question is a bit vague, but have you looked into Google Picasa? It has a function that allows you to pick your pictures and automatically put them into a collage. Is is pretty basic, but a pretty good starting point.

This function is under the "Create", "Picture Collage" menu item.