You should send 2 messages.
1. Firstly a mathematical message in order to catch there attention, detailing Prime Numbers:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, Why?, There is no naturally occurring prime number generator. An Alien race would know for certain the message is from a life form.
2. Second message, much more details such as how many planets our system has, our location being 3rd from the sun, population of the earth, a brief description of our evolutionary path, greetings in all languages, songs from the Whales and some music, being The Earths Greatest Hits!
The list is endless,