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Let's say your from America... you could say:

Watashi wa Amerika-jin desu.
わたし　わ　あめりかーじん　です。
(I am an American)

Please don't just say "Amerika" if you only say "Amerika" that means that you are America. That's what "-Jin" is for. "-Jin", you could add it to the end of any country's name in japanese and it automatically means that who ever you are talking about comes from that place or is a residence.

Here are some other countries that so far i know:

Japan :: Nihon :: にほん

Canada :: Kanada ::　かなだ

Australia :: Osutoraria :: おすとらりあ

Brazil :: Burajiru :: ぶらじる

India :: Indo :: いんど

China :: Chugoku :: ちゅごく

England :: Igirisu :: いぎりす

And if you want to ask someone if a certain person is this or that, you say:

Person's name (including -san -kun -chan) wa Amerika-Jin desu ka?
Person's name (including -san -kun -chan) わ　あめりかーじん　です　か？
(Is Person's name an american?)

The person your asking could say:

Hai, Person's name (including -san -kun -chan) wa Amerika-jin desu.
はい、Person's name (including -san -kun -chan) わ　あめりかーじん　です。
(Yes, Person's name is American)
or

Iie, Person's name (including -san -kun -chan) Amerika-jin dewa arimasen.
いいえ、Person's name (including -san -kun -chan)　あめりかーじん　でわ　ありません・
(No, Person's name is not american)