I am really pleased that your family will soon be joining you in Germany. If you are still living in municipal accommodation yourself, go to your job centre. Usually, the job centre is informed that you have applied for family reunification. Tell them the exact date of their arrival so that the city can prepare accommodation for all of you.
If you already live in your own flat, **** for a larger flat yourself. Depending on whether you receive social benefits or do not earn enough money for a larger flat yourself, you should also clarify the financing with the job centre. Discuss with your job centre how big the flat can be and how much it can cost (this depends on the number of people). If you receive financial support from the Job Centre, get a written statement that the costs will be covered. It is helpful when looking for a flat if you can already show potential landlords a written cost transfer from the Job Centre.
I wish you and your family all the best.