Capitalize "Americans" when:
* It refers to the people of the United States of America: "The Americans are known for their love of freedom."
* It's used as a proper noun: "The Americans and the British fought in the Revolutionary War."
* It's part of a formal title: "The American Embassy in London."
Don't capitalize "Americans" when:
* It refers to people from the Americas in general: "Many Americans are bilingual."
* It's used as a general term: "Americans are often stereotyped as being loud and friendly."
In doubt, consider if you could substitute "Americans" with "people of the United States" or "citizens of the United States." If you can, then capitalize it. If not, leave it lowercase.