Hi Andrea,
I found the Generator hostel fantastic. The location was brilliant, right next to the metro station, on the main circle lines. There was a tram that stopped around the corner to take you to alexanderplatz, and the best thing is I never brought a ticket! I probably shouldnt admit it, but its true!
One huge tip for travelling in Germany, I dont know if you have heard of a website called Mitfahrgelegenheit. Its a carshare website, its 100% safe, I was travelling alone and did it lots of times. You have a look where you want to travel to and from (eg, Berlin to Dresden) on a certain date, and it tells you all the people who are driving that route on that day who have spare seats in their car, and it only cost about €5-€10 each.
Have fun travelling!
Jason