You will have to apply for a visa, which is quite easy to get.
umm by a plane
Not if you are a US citizen. If you are a citizen of the country, you can travel to any place part of the country without a passport.
Denmark
You don't need a passport to travel within the continental United States, even if you are not a citizen.
You can if you are a US Citizen and are entering from the US.
Andaman and nikobar
Yes, they can because Slovakia is part of European Union and Schenghen too.
As a British Citizen you are able to travel freely within the boundaries of the EU, without requiring further visa's.
Only if you can prove you are a US citizen or are entitled to be one.
No
If you are a Spanish National or a citizen of another European country then you do not need a visa to travel to Ireland. Anyone who holds a passport and is a citizen of the European Union is free to travel anywhere within the union.
Since the UK and Italy are both a part of the European Union, you can travel visa-free to the UK if you are an Italian citizen (or vice versa).