"IIe, watashi wa nihonjin dewa arimasen." if you want to be polite(it means "I'm not Japanese"
"Iie, watashi wa nihonjin dewanai" if you want to be casual, you can change watashi to ore or other form of "I" but they are not polite.
and if you want to communicate that you are american just add after that "amerikajin desu"
Chigau desu might be right too but its more like "Its false" or "Its wrong" or "You are wrong" depends on context.
ps. no idea of there is space between dewa and nai, in kana/kanji there are no spaces.