So now I have an apartment and Bluecard(residence card), feels like I am settled.
I read excellent article - 'How to find a flat in Berlin' by Jon Worth just before I came here, which became very helpful during my search. There are few websites for apartment listing, one has to contact and arrange appointment to visit flat. Usually it's mass viewing i.e agents invite all the people who are looking for flat at same time. In one flat I visited, there were around 25 people who came to visit.
After viewing, interested people fill form with basic details, monthly income etc. Then agent selects tenant based on above provided information. These all sounds like very competitive. For quite a long, it seems Berlin was known for low rents but not anymore as demand is very high. I visited three flats, got selected by one of them. At the end, I took a flat from a colleague who was moving to some other place.
I also received Bluecard on third week of September, just two months after I arrived in Germany. I had to go to Foreign Registration Office in Berlin, submit my documents(same documents which were submitted to get work permit, and visa). D category visa allows to work, even though my residence card was processing (so I started work a day after I arrived here :D). I had to pay 110 € as application fee, and they also provide a paper which is temporary residence permit.
After about three weeks, they said they would send a letter by post office. That letter never came in my case, which was supposed to let me know status of my application. So, I went there without it, and my residence card was already there ready to be picked up. It was faster than expected :)
So that's that. Now I really want to explore Berlin tech communities, which I haven't done because of above things. I am looking forward to my first Django Meetup. Already feeling jazzy ;)