I realized that this topic is discussed again and again, so I want to give you short but important information about getting a prior approval for your visa/work permit!
The prior approval will shorten the whole application process immensely!

1. The employer has to apply for it!
2. If your salary will be less than 47.600,- € per year the ZAV has to be involved (Central Employment Agency)
3. If your salary is above 47.600,- € the ZAV is not important any more and your future employer can apply directly at his local foreigner office for a so called 'Vorabzustimmung'.
4. The employer needs the employment contract, your passport copy and a copy of your University degree. This should do it.
5. Documents needed for ZAV approval you will find when you follow the link below
http://www.arbeitsagentur.de/
Good luck
Detlef