How do I prove that I am a US citizen?

Hi my name is Leonel Thompson and fall in the category of the child act of 2000 and I met all these requirements of The following are the Act's requirements: 1. At least one parent of the child is a U.S. citizen, either by birth or naturalization. 2. The child is under the age of 18. 3. The child must be residing in the United States in the legal and physical custody of the U.S. citizen parent after having been lawfully admitted into this country as an immigrant for lawful permanent residence. 4. If the child has been adopted, the adoption must be final. I want to know how do I get proof that I am a US citizen? Do I have to fill out some type of form? And when I applied for a passport it asked for a proof of US citizenship can you please help thank you.