Are you a Lawful Permanent Resident and want to petition your single child who is under 21 years old? In this case, you have your Green Card and it will be considered a Second Preference Petition and you can move on to the next step the moment it becomes current to either adjusting status or consulate processing.
The first part must be done and must be done properly in order to ever proceed to the next part. Without a proper I-130 application there will never be a Green Card. If approved, then you could move onto the next step in the Immigration process. If not approved, it will take years of red tape. You might be able to 'lock' your child's age in as younger than 21 as well. Remember, it is your responsibility to submit everything correctly.
This sample I-130 Petition has everything necessary to give you the best chance possible to get an approval. Just input your own information and use the application as a guide. It has been prepared by an expert Immigration Attorney. Thus, this book will allow you to see a guide of how it is done, prepared and submitted.