Losing the things you care about in a new country is terrifying!
There are several ways to track your stuff and a list of things you should or you should not do. 

The most important thing is patience. Don’t panic, don’t lose patience. Keep calm and start evaluating the situation and what should be the most reasonable next step. Make a list of all the things which you have lost. If possible, document the incident. It will be easier for you to track what has happened and for the authorities as well to figure out the next course of action. 

If you have forgotten or lost your stuff in trains, trams or buses or anywhere else,

* Contact BVG or DB depending on the train or bus you were in. Inform them about the time and the station from where you boarded and where you left. 
* They will immediately check with the driver if he or any of passenger has found anything and you will get your stuff back. Either you can collect it from the driver or you can go to the respective lost and found office and can take it from there. 
* Usually, you might not get your lost stuff on the same day, as the drivers might give it to the lost and found office at the end of their working shift.

If your stuff is stolen, 

First thing is to do a police complaint, especially if you have lost something important like a passport or any other document. To get a new one you will need a copy of the police complaint. After the police complaint, report the embassy immediately. In stolen stuff, there is hardly anything else you can do, you will give the details to the police and they will do the rest. In some cases, things like documents which are not sellable, they throw it on the streets or dustbins and you might get them back.  But, it’s not for sure. 

Things you should do if you are traveling in Berlin,

Though Berlin is a safe city few areas like Kreuzberg, near Warschauer Straße station, Neukölln, Wedding and especially in any sort of public gathering be extra careful about your belongings. Please keep a copy of all your important documents, especially your passport. 

If any stranger tries to be friendly with you, be friendly to the person but don’t trust them with your stuff. You can find a lot of people and stories in which a person was being nice and friendly and later took something from someone’s bag or stole a phone or wallet. 

Here is the list of all the Lost and Found Departments in Berlin. 

* Zentrales Fundbüro (General Lost & Found) Platz der Luftbrücke 6 12101 Berlin-Tempelhof Telephone 030/90277 3101
* BVG-Fundbüro (Public Transport Lost & Found) Potsdamer Str. 180/182 (Nähe Kleistpark) 10784 Berlin BVG-Callcenter: 030/19 449
* Lost bicycles at the Deutsche Bahn/S-Bahn: Bahnhof Zoo Office Telephone: 030/29 74 93 22
* Lost Animals Tiersammelstelle (on the premises of the animal shelter) Hausvaterweg 39 13057 Berlin (Falkenberg) Telephone 030/76 888-201 or -203
* Tegel Airport (Lost-and-Found-Service) There is a combined counter for lost-and-found items, left baggage and for requesting a baggage handler. This counter is located in the Airport Service Center in the central hall, telephone: 030/41 01 23 15
* Schönefeld Airport (Fundbüro Beyer Sprach- & Airport Service), Terminal A, ground floor, Tel. 030/34 39 75 33
* S-Bahn Berlin Rudolfstraße 1-8, 10245 Berlin ,  +49 30 297 43 333 

You can check an article about all the emergency numbers you need to know when you are in Germany here: Emergency numbers in Germany

In case if you need any help in writing police complaints in German, or an interpreter for your visit to the police station or any office, you can share your request on MyHelpBuddy and book a native speaker who could assist you.