Hey there! I started my transition in Berlin as well and I'm an American. Go to Dr. Sieglinde Bast to get your psychological evaluation out of the way. She doesn't really ask questions, all you have to do is give her a full written account of your feelings and why you want to transition, and she'll send you the confirmation so that you can get testosterone.
Meike Mentzel is also a very good English speaking therapist who can give you this as well if you want to work with her long term, but if you just want the piece of paper, go to Dr. Bast.
The best doctor in town for to get your hormones checked and given is Dr. Christoph Schuler who works at Praxis-Turmstraße in Moabit. He's incredible and very sympathetic. He deals with a lot of trans* people. It helps if you speak some German, but if not it's still ok. Try to find a friend to go with you if you don't speak German.
If you want a more extensive list of doctors and practices, go to TransInterQueer (TRIQ) and they can help you.