If you are a US citizen and if you are genetically related to the child - have the same DNA, then you can go to the US embassy and file Consular Report for Birth Abroad.
If you are married to the child's mother and are a US citizen, you can file a family petition - one for your wife and a second for her child as the step parent.
I am sorry I cannot provide more guidance, but your question is confusing - I don't know who you are in this situation.
I strongly recommend that you seek the advice of an immigration attorney in Texas. Do not throw your money away by going to a notario.
Best of luck