I grew up in the UK with a little exposure to the US via various media sources. I traveled to the US for the first time when I was 17 and visited Los Angeles. I finally moved to the US when I was 20 and lived in Dallas.
My main impression of the US, based on everything from the Moon Landing to my personal experiences in LA and Dallas, was that an American is someone with a "can-do" attitude. As a newly-minted US citizen, this is certainly something that I intend to exhibit to my dying day.