Student or Learner
I would like to know how to adress to an undetermined person in a formal letter, such as the members of th U.S embassy?
Dear Sir or Madam
is the most common and won't get you into trouble, but Americans are always more impressed if you are polite and find out who exactly will read your letter, and then use their real name, as in "Dear Mr. Clinton".
Incidentally, sebitas, it may help you to remember the spelling of 'address' if you think of the Latin for Castilian a: it's ad.