1) If you come from a country member of the European Community you shouldn't have any problems working in Greece.
2) Register to the local police eight days within your arrival.
3) After that you have up to three months to work or spend time in the country.
4) If you intend to work serving drinks or food you will be required to get a "health-book" which is a document you acquire through a medical exam.
5) Make sure you do not have TBC or Hepatitis.
6) For more than three months you need a residence permit.
7) You can get a six month temporary permit with the local police. They will need two passport photographs and your passport to register you.
8) If you decide to stay longer than that, the next permit covers a five year period, and for this you will usually be asked to do a medical exam, which basically is the same one as described above.
Some piece of advice
Ask your employer to pay you on a daily or weekly basis.
He is also to obliged by law to pay your social security called IKA in Greek (500 € monthly).
He will also have to give you a Christmas and Easter bonus equivalent to the time you've worked for him.
Before starting to search for a job or getting one, ask foreign workers to tell you give you some good advice on where to look and who to avoid.