You have discovered, as so many before you, that the most difficult part of writing is getting started! The first thing to do is to tell yourself, "I'm just going to write some stuff down, and it doesn't have to be very good." Then--start writing! You can always improve it later.
Now then, let's look at the assignment itself. I assume that "footprints" is meant metaphorically and that the subject to be addressed is how one person can make an impact on a big world. One way to approach it would be to think of people who have done just that. Who comes to mind? Mother Theresa certainly helped a lot of poor people, and set an example for millions. Rosa Parks refused to give up her seat on a bus and helped change segregation laws in the U.S. Presidents and Prime Ministers, of course, create events which can change the world. One could also think on a more local level, perhaps someone you know or have read about in your community. Using examples is the best way I can think of to prove your point. See if you can think of a few, and that may help you get started.
I hope this helps!