Do I need to have my Bachelor Certificate at the time of applying?
No. You can also apply, if you haven´t completed your Bachelor's degree yet. It is enough to submit a "Zwei-Drittel-Bescheinigung" (a document certifying that you´ve completed two thirds of your bachelor studies). Alternatively you can also submit a transcript of records which shows that you have completed two thirds of your bachelor studies.
Without having completed your bachelor studies you can get enrolled temporarily with the condition of submitting the certificate of your completed bachelor degree until the end of the first semester (january).