You are required to obtain a Tax ID number in order to file a tax return. In most of the cases this would be the SSN issued to you. The purpose of the social security card is for employment authorization under au pair regulations. If you have not already obtained a social security card, please review the information linked below to make sure to apply for one. You may also check with your au pair service provider to see if they have a contact that assists with obtaining SSNs. You can get started with filing your return if you do not have one, but you will need the number before the tax return is able to be submitted to the IRS.
Information on obtaining a Social Security Number
https://www.ssa.gov/ssnumber/