Hi Albi,
I can't help you out with offering a room but here are the websites where you could try to find one:
Shared Appartements in Munich at Studenten-WG.deThese rooms are directly offered by students living in shared apartments.
You could also search at
Homecompany Munich, a company, that specializes in furnished rooms for short term rental - but they charge a commission fee if you rent a room thru them.
Unfortunately, Munich is the about most expensive town in Germany regarding rent for flats or rooms.
As you are looking for a room for the summer time when no university studies are taking place (or less than usual), you could also ask at the
Studentenwerk Munich (in English :) ) for a room in a student dormitory for the summer time. On the site of the Studentenwerk (a "Studentenwerk" organizes and supports student life by offering dormitories, dining facilities for students, etc.). you find much more information about renting a room in Munich (all in English).
If you don't find anything via the said links, you'll have to stay at a
youth hostel in Munich first and stroll around munich university looking for bulletin boards and in student bars across town to look for rooms advertised there.
Good luck
Chris