A dual citizen is one who holds legal citizen status in two different countries. Dual citizenship is obtained in two ways: transferred citizenships during your birth or by naturalizing in another country. The laws of both countries must allow for dual citizenship, some countries force you to renounce all other citizenships. Benefits and detractions apply to holding dual citizenship, and acquiring dual citizenship shouldn't be done without heavy consideration.
Dual Citizenship at Birth
Be born abroad and register the birth with the closest embassy or consulate. Most countries grant citizenship to those born inside their country. Most countries allow for the transfer of citizenship from parents to children born abroad --- the child will acquire the citizenship of both the country he was born in and the citizenship of his parents.
Take the birth certificate to the nearest embassy or consulate of the country in which the parents hold citizenship.
Register the birth abroad with the embassy or consulate. Provide any required documents including proof of citizenship, birth certificates and marriage licenses. The birth certificate will then be proof of citizenship for both the country it was issued in and the country the parents are citizens of.
Naturalizing in Another Country
Enter the desired foreign country legally and apply for permanent residency status. Contact your nearest embassy or consulate to gather information on how to do this and apply for permanent residency in the appropriate manner.
Move to the desired country and physically reside there for the appropriate amount of time. Most countries require a physical presence in the country for a number of years.
Apply for naturalization with the country's department of immigration after the appropriate amount of time. Contact the country's department of immigration for instructions on how to apply.
Provide documents required including birth certificates, proof of port of entry and original entry visa. Pay applicable fees and cooperate with the government. You may be asked to take a test, swear an oath and demonstrate a proficiency in the local language.
Obtain your naturalization papers to become a dual citizen.