What are the steps in applying for child under 21?

You should first determine if the child has a claim to US citizenship as an I-130 will be denied if such a claim potentially exists. It depends on how long you have lived in the US. If a claim exists, then you should submit a US passport application on your child's behalf at the US Consulate. You will also need to file a Consular Report of Birth Abroad. If there is no claim, then you can proceed with the I-130. You will need to include your proof of US citizenship, your marriage certificate, and the child's birth certificate with translation, if necessary. Normally, that is enough to get an I-130 approved.

YEs, you need to petition for your child now and once the child enters the US as a green card holder, he will also automjatically be eligible for US citizenship. You will need to submit the I-864 for your son but that will be at the consular processing stage and not the initial I-130 stage.

If you were a US citizen and married to his mother when your son was born, he may already be a US citizen by birth if you had resided in the US or its possessions for at least 5 years prior to the child's birth, two of which were after the age of 14. The rules are different if you were not married at the time. Otherwise, you will need to submit an I-130 petition for him along with proof of your citizenship, marriage certificate, any divorce decrees, and his birth certificate.

If you have been a US citizen for more than 5 years and resided in the US for 5 years before the birth, your child may already be a US citizen. Check with the consulate about the procedures to get the child a US passport. Otherwise, you have to fie a new I-130 for your child.