Alien Registration Number, or shortly A-Number, is an eight- or nine-digit number issued by USCIS to permanent residents and certain classes of nonimmigrants. It is obligatory to indicate A-Number in all USCIS forms, if you have it. And in case you were not granted an A-Number, simply leave the corresponding field of I-130 Form blank.
You, as a petitioner, should indicate your Alien Registration Number, if any, in item 1 of Part 2. “Information About You (Petitioner)”. And in item 1 of Part 4. “Information About Beneficiary” you should insert A-Number of your beneficiary, if any. Please note that if the A-Number is shorter than nine digits, it is required to insert zero after “A” and before the first digit.
If you are not sure where to find the Alien Registration Number, look through the following documents issued by USCIS: green card, immigrant visa, Employment Authorization Document (EAD), Notice of Action, Immigrant Data Summary, Immigrant Fee Handout.