Admittedly, the title of this thread made me smile a bit. To answer the questions within however:
1. What is your religion when you were born?
I was born into a Christian family, and I still live in a predominantly Christian region.
2. What do you really believe? Science?
Yes, I believe in science. However, I think an atheist saying that he/she believes in purely science is but scratching the surface of their beliefs. I also don't think that anyone's beliefs (atheist, or otherwise) can be summed up in one word or phrase. I believe in logic, science, reason, truth, beauty, inquiry, and empathy. I believe in all of the problems, mysteries, and absurdities of life. The "a" in "a-theist" does mean without, but being an atheist does not mean being devoid of belief. The major difference therein lies in the lack of doctrine. Each atheist must pursue his or her own beliefs. Their attitudes toward religion, life, and themselves will help define their beliefs.
3. And, when and how did you became an Atheist?
This is a difficult question for me to answer. In fact, it's probably the most difficult question for me to answer out of the three. I will give you a brief summary. I first doubted God at a very young age. This was at a time when I did not know the word "atheist" even existed, much less the idea behind the word. This was at a time where there was no other option aside from Christianity and a belief in God. I was naive and inexperienced. I do not blame my family or environment for that. I was simply young. It would take time for me to come to terms with my lack of belief in God and even begin referring to myself as an "atheist." I struggled with it a lot for several years, but the more that I read and the more people I met, the easier it became.