If you have a name, then use it. When you say you do not know if it is a man or a woman, does that mean you only have a surname? If so then you could say Dear Mr / Ms xxxxxx.
If there is no name, and this is your initial approach, then you do not need a salutation. Your e-mail should start with the statement "With reference to your advertisement in XXXXX for [position], I would like to apply for this." Then give very brief information and ask for your application to be considered, and for any information on the job to be sent to you.