As Michael said in the comment, the commonest way is...

I lived in America for 10 years.

Or, say as you said...

I was in American for 10 years.

Alternatively,

I had spent 10 years in America

As Michael said in the comment, the most common way is:

I lived in America for 10 years.

Or, you can say as you said:

I was in America for 10 years.

Alternatively:

I had spent 10 years in America.