America is a HUGE and diverse country with many reagions and ethnicities and cultures so you really need to narrow your question to a region. Jeans are fairly popular, but not everone wears them but they were invented in America so and are popular so i suppose you could say jeans. Tho there are people who dress fancy and flashy, in general, American is fairly casual and American fashion changes fast and often too. It would be much easier if you chose a specific region like the Midwest or Deep South, Upper South, New England, Pacific Northwest, South West, Border, West Coast, Wine Country, North, Great Lakes, Gulf Coast, Louisiana, Bayou, New York, Los Angeles, Hollywood, etc. Cultural American fashion will be different one place to another.