23 year old looking for cool flatshare in Berlin

Hello Berlin!
As of Saturday, I am a 23 year old English/German student in the UK. I’m coming to Berlin to spend most of my summer holiday learning to speak German and getting to know the country. I have visited Berlin before (a couple of months back), and am really looking forward to experiencing the city again.
That said, I’m no expert, so my ideal situation would be a flatshare with some young people in a relatively active and energetic district! Definitely not looking for a professional flatshare arrangement, instead I’d prefer to live with friends, in a flat that is for socialising as well - everyone likes to get a beer, watch some films, etc.
Obviously, I’m not looking for the nicest flat in the world, but somewhere to hang clothes and study (when necessary) would be nice. Clean and functional is nice, as well as simple decoration. I will naturally need a furnished room.
My budget is up to 400 Euros per month (perhaps higher if necessary), but I’m also happy to eat together, and I can cook a little. I’m looking from now until September/October, if things go well.
I also enjoy going to the gym, rock climbing, and playing piano. I am happy to Skype.
I hope to hear from you all soon!

Luca