The most common website for flat offers and requests; also available in English, highly recommended:

http://www.wg-gesucht.de

 

Made by student government (AStA) of TU Dortmund and FH Dortmund; only in German and with a significantly lower number of flat offers:

http://jobsundwohnungen.asta-dortmund.de

https://www.studierbar.de/?page_id=208

 

There are also some facebook groups like...

WG & Wohnung Dortmund

TU Dortmund WG Börse

 

Also possible, but not recommended for short-term stay:

http://kleinanzeigen.ebay.de/anzeigen/s-wohnung-mieten/dortmund

http://immo.ruhrnachrichten.de/mieten/dortmund

http://www.immobilienscout24.de (watch out, they often charge you extra commission!)

 

Also usefull Links, which you should check out:

http://www.studying-in-germany.org/ -- Which contains detailed and accurate information about Studying in Germany, this website is also translated into 10 different languages.

 

http://www.germany-visa.org/student-visa/ -- contains detailed information about Germany Student Visa and requirements

 
http://www.germanyhis.com/health-insurance-students/ -- contains detailed information about Germany Health Insurance      for Students  
 
https://www.mastersdegree.net/masters-degree-germany/ - This site contains very useful and detailed information about   studying Masters in Germany.
 
https://visaguide.world/europe/germany-visa/who-needs-visa/ - Here you can check if you need a visa to enter Germany. This side also contains how to apply for a german visa and the different types of existing visa.