Ausbildung after completing an Au Pair year

Ask for the requirements at your nearest German embassy!
To do an apprenticeship (Ausbildung), you need to find a company willing to take you as trainee first.

I found one and they told me i need to find a Berufsschule. Will I get a place with my “not soo good” German, as Dentist Assistant?

This depends largely on the clinic you apply at. Some (I suspect most) would certainly prefer assistants who can understand patients and instructions given by the dentist. Others might be willing to give you a chance - especially if you can show that you are hard-working and motivated to improve your language skills. In that respect, I think it would be good if you can show at least a basic certificate (A1 or A2) and that you are continuing the classes for the next level(s).
The Berufsschule probably has a language requirement for entry. This is often B1 or B2. Inquire at the school - and also ask whether it is possible to do extra language classes to achieve this within the first few months of Berufsschule.
Good luck!

Thank you soo much. Is it also possible to do your Ausbildung then later Studium?

"Studium" usually refers to university studies leading to a degree - and an "Ausbildung" (apprenticeship) is commonly seen as alternative to a degree for those who cannot make the entry requirements. It rarely makes sense to to both - and a completed apprenticeship does not make one eligible to enter university.