If I have a baby that is a citizen of the US but I do not have any legal documents, what papers do I need to get a green card?

Unfortunately, not until the baby turns 21... And presuming you entered the US legally. How long have you been here?

If you entered without inspection you will not be able to process. In addition, you will need to wait until the child turns 21 to file for you. There is an existing exception that permits the filing of a penalty ($1,000), if you have had a previous family or employment based case filed by 04/30/2001. Has anyone in your family filed such a case? Otherwise, you must go through a consulate process and be subject to a 10 year bar.