An Alien receipt number (or A-number) is assigned to you if you have ever applied for any type of immigration status, if you were ever in removal procedings, or if you were aprehended within the US, not necessarily if you were apprehended at the border. If you don't know if you have an A-number, you could ask you parents/guardian with whom you came over. If you don't have your A-number, you just put on the back your name "LAST NAME, First name" and your Date of birth. So you did the right thing.
[sources from Atlasdiy on youtube found here << http://youtu.be/khXvPQdhZLc >>At the 3:33 minute mark, she starts talking about the A number. You would do well to watch all those videos ]