Yes. Every letter mailed in the US needs a stamp.
No, you would need to get a stamp for the country the letter is mailed from.
What is the cost of a letter to be mailed from US to Canada
The stamp has to be accepted where you mail the letter. If you are mailing in Canada, you need a Canadian stamp. If you are in the US mailing to Canada, you can use a US stamp.
What is the current cost of a stamp on a first class letter under an ounce mailed in the US? <><><> 44 cents
How much is first class postage from US to Poland?
One 41 cent stamp for a letter of no more than 5 or 6 pages if it is mailed ANYWHERE in the US, until the rate goes up to 42 cents in May 2008.
The First Class US postage rates (not including Priority Mail) are the same to and from anywhere in the US. For a one ounce regular-size letter mailed in May 2009, you need 44 cents in stamps or one Forever Stamp. Additional ounces are 17 cents for letters up to 3.5 ounces. Note: The rates change every year. The above rates were current when this answer was written.
Only if mailed from England to the US. You can't mail in the US using a foreign stamp, but must have US postage.
Are you talking about email or mail sent with the US Post Office?
Depends on the weight of the letter. For an ordinary letter, one first class stamp will do. Weight, not distance.
No, you cannot do this. Stamps used to mail a letter have to be those of the country where the letter is mailed. The International Postal Union helps with the regulation and trading of mail between countries and appropriate compensation.
A regular letter costs $1.05 from the US to Luxumburg See: http://postcalc.usps.com/