In Japan, rice and noodles are the most common staples.
In America, corn, potatoes, and bread are the most common staples.
In Japan, fish is the most popular choice of protein, due to Japan's coastline,
while in America, meat is more popular due to the larger land mass that it has.
American cooking favors the use of strong tasting condiments like barbecue sauce, ketchup and Worcestershire sauce. While in Japan, most condiments are relatively mild.
In America, salt and pepper as well as meat(chicken or beef) stocks are the most commonly used for flavour whereas in Japan, soy sauce and dashi (a fish, kelp or mushroom stock) is most often used.
Baking is very often seen in American cuisine. In traditional Japanese cuisine however, ovens are not often used.
There are many more differences between both cuisines. I encourage you to try both to find out.