Tax Number

The tax authority (Finanzamt) should have sent you an official letter with your tax ID soon after you registered your first residence in Germany. If not, call them and ask to have it sent again!
Your employer will also need your social security number (Sozialversicherungsnummer) and proof of your membership in a health insurance.

Deppiiii thank you a lot 
yes, all the other documents are finished. 

Please do not mis-spell my name like this: "Depp" means idiot in Bavarian dialect!

lol- I'll call you that in future Beppi.

@ Maria,  probably the only thing that I can add from above is that you have to be more specific on "they" in your message. Some "they's" needed something and other "they" gave you something. It difficult to know whether "they" gave you accurate information.

As stated above, there is a regional finanzamt that can either send you a letter or tell you what it is (11 digits).Just make sure it's not the eTIN that the employer usually have for your salary slip (it's something different).