Americans like to smile a lot, even at strangers, and this becomes more true the older the person.
An informal attitude prevails among most Americans; do not take this as their being disrespectful.
Shaking hands in a social environment is not always required, especially if it is between opposite sexes. However, if you do, make sure you hold eye contact.
Holding the middle finger up by itself is considered a rude and obscene gesture. Be careful, as many non-Americans tend to point or make gestures with their middle finger alone without thinking.

https://www.cia.gov/library/publications/the-world-factbook/geos/us.html (Majority of information)
http://www.census.gov/prod/cen2010/briefs/c2010br-02.pdf (Ethnic data)
https://factfinder.census.gov/faces/tableservices/jsf/pages/productview.xhtml?src=bkmk (City population data)
Wolfram|Alpha (Currency exchange data)
Information accurate to best of ability as of 26 December 2016
¹Those of Hispanic or Latino background (denoted primarily as White or Black) have heritage from Latin America, South America, and/or the Caribbean. They make up 16.3% of Americans.
²Includes native Alaskans
³Includes native Hawaiians