Manners maketh man, so they say. Of course, manners are highly context dependent. What's polite in one culture may be considered rude in another. For example, there are some places in which it's polite to refuse a gift the first time so as not to appear too eager, only for the giver to insist. In others, that would be considered dismissive on the part of the recipient and pushy on the part of the giver. In some cultures, it's considered polite to clear your plate, to show that you really enjoyed your food. In others, this implies that you think your hosts have underfed you and is very rude!
A lot of American ideas about politeness are aligned across the entire country - but not all! In some areas, minding bread-and-butter etiquettes such as thank you notes, remembering P's and Q's, and dressing appropriately is very important. In others, treating someone's time respectfully by keeping it brief and efficient is considered paramount.
This depends on a lot of factors, from education to class to whether you're in a rural or urban area - but you can also assess it by state and region. That means if you tell us about your manners, we can point you to a place that they will surely be appreciated. Why learn to be polite, after all, if you can find a place that you'd already be considered polite?