help please i dont speak english very well i need help with a child game face

Hello my english is very poor, for that i can´t search in the forums.
well my problem is i have a child, and i cant create a game face for him, because i can´t isert in the Web with his account because when i try to create a new account the website said "already exist account with that e-mail", when i try "i forgot my pasword" it said "no exist account". And i have authorised already to him for playing online and i try to do a new person in my account with his gamertag it said "that gamertag already exist".
Ah!!!!!!!! i have xbox 360