Adoption

Hello, I am 16 year old Cameroonian, I want to be adopted by a US citizen, but it is complicated and i have some questions and i would really be distressed if you answer me personally;
1) My dad is dead and my mum is married to another man with 4 children, and this making me have a stepdad, but she and her husband are still surviving parents, who are so poor that they cant finance the needs of their own child, so what about me. So i wanted to know if tho my mum is wedded, i am still defined as an orphan under US law or can i still get adopted normally?
2) Do I need my mum's concern to get adopted, since i am 16, the person who wants to adopt want that we should do it under US Law, so i will first have to immigrate to the US with and get adopted there, this is possible? yes, how?