@Marta1150
If you're looking for a traveling partner to accompany you during your stay, you'll get the most traction on social media networks like F***book. You could also put an ad on https://www.tripgiraffe.com/ .
If you'd like to rent a furnished guestroom in a shared apartment, use the "Private Room" filter in your Airbnb search. Bear in mind, most Berlin hosts are not currently welcoming strangers into our homes due to the pandemic.
For a longer term rental in a shared home, the most popular site to search is called WG Gesucht