No question is a silly question!
Ok, usually when writing an important letter to a person you don't know (and you don't know whether the person is a man or a woman) you should start your letter with:
Dear Sir/Madam, or Dear Sir or Madam,
If you know the name of the person you are writing to, always use their surname. For example, you're writing to a 30 year old man called John Smith, you would write:
Dear Mr Smith,
If you are writing to a very young man, for example a John Smith who is 12 years old, you would address the letter:
Dear Master Smith,
Women are slightly more complicated: (In more ways than one! Maj - please don't be too angry!)
Jane Smith is 30 and we don't know whether she is married or not:
Dear Ms Smith,
Jane Smith is 12 -
Dear Miss Smith,
Jane Smith is married -
Dear Mrs Smith.
The problem is that some women that are married do not like to be addressed to as Mrs and like to remain Ms.
Therefore if you DO NOT know whether the person is married or not,
just write Ms.
Hope I've been of help,
Chris