I would say, "Look at me!" ;)
No, in all seriousness, I guess first I would see if there was a way to break the language barrier. Perhaps they cannot grasp things in human terms. I'm always amazed that all the movies show no imagination. Do you notice that all the aliens still somehow have some kind of eyes, ears, nose, mouth, etc? I think if we find extra terrestrial life, why should it be limited to things we understand in human terms? Other life forms don't necessarily have to be tied to the human laws that bind us. Perhaps they react differently to the laws of the universe.
Anyway, so first you would have to find out a way of communicating (if that's possible)... and then maybe you act out something as in charades. "I'm from over there... and time it right so you're pointing to earth..." and most likely by the time science comes up with a way to travel to mars, hopefully we will have better telescopes where we can see it much more vividly to show them. I think they would learn more by observing us than we could tell them.
Unless of course, they end up making it here first. In that case, hopefully they don't get attacked or dissected first, but if not, then we show them pictures of their planet and most likely they would recognize it. Then we could proceed to show them our planet, and in so doing, they would get a picture of what humans are.