Once you move to Germany and take up employment or start earning money you will need a tax number from the tax office. Here is how you can get your tax number.
The short answer
Once you register your residence for the first time in Germany, the tax office will send you your tax number within a few weeks. This is the number you will have to provide to your employer.
Below is a sample of the letter you receive. I have attached an English translation.
The longer answer
You may already know this but there is a difference between a tax ID(Steueridentifikationsnummer) and a tax number(Steuernummer). The tax ID never changes as stated in the letter above.
The tax number however may change. When you move to a new district, town, or city, the tax office at your new residence will write to you after a few days with a new tax number. They will also tell you to discontinue using the old tax number with immediate effect.
Can’t find your tax number? This is how you find it
If you lose your tax ID here is a checklist of how you could retrieve it.
You can also call your tax office directly. It usually takes weeks for you to get this. You can save a lot of time looking through old documents and keeping your tax number safe as soon as you get it.