For general information, you can for example take a look at the following websites (mostly in German!):

www2.dgb-jugend.de
www.gutefrage.net
www.123recht.net

If you are really having problems, you can get legal advice for free at:

International Office
Studierendenwerk Ulm - if it is within the remit of the Studentenwerk
Deutscher Gewerkschaftsbund - labour law for jobs or work placements during studies

Internet forums and blogs on rental law (only in German!)

www.mietrecht-hilfe.de
www.mietrechtslexikon.de
www.helpster.de

Templates for dismissals and other „official“ documents:
(mobile phone, flat, health insurance etc. (mostly in German!))

http://www.scribd.com
http://www.briefform.de
http://www.vorlage-musterbriefe.de
http://www.arbeitshilfen-online.de