how to get a work permit (Berlin)

Hello Fellow expats,

I got a job offer from a company in Berlin, and as US citizen I need to have a work permit before I start working.

so I would appreciate if anyone can answer the following questions.
1- how long does it take to get a work permit.
2- is there agencies in Germany that can help expedite the process or at least do it for me before I come.
3- can I work after I apply for the permit while its being processed or do I have to wait until the permit is issued!