Working in other Europe for a Ukrainian student

I am a 17-year-old Ukrainian student; I thought about working somewhere in Europe this summer. There are strict rules about documents in Germany, as I've heard. Can I just go, theoretically, to Austria and just find a work through websites? I know both German and English well.

If you can advise, any other country to work in.

Also, if you know any legal regulations, I'd like to know them as well. I heard I can work only 90 days without any contracts, with my Karticka pobytu, could anyone confirm?..

Or, maybe a good way to find work there(agencies included)