First you need a job. Your future boss will provide you with a description of the job. This will describe the conditions for you getting employed. You then take this letter to the immigration authority ("Ausländerbehörde"). This is normally located in the community where you are accommodated. You submit your application for a work permit there.
Language, language, language
This will in turn take some time, because a check is made on whether you will work under approximately the same conditions and for the same money – as others – for the same work at the same company. The authorities will be speedy in issuing you a permit as quickly as possible, because in general new employees are needed in many areas. However, it will also be checked that there are not unemployed German job seekers who are suitable for this job and that you have the proper qualifications. Here too the principle applies that the better your command of German, the better your chances are of a good job.