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Munich is the most difficult (and expensive) city in Germany to find rental accommodation.
Since no trustworthy landlord would agree to give you a place without having met you - and also you should never agree to rent something you didn't visit and inspect - you can effectively only start your search for accommodation after you arrive.
Therefore, you should book a temporary place (hotel, hostel, B&B or serviced short-term apartment) for the first few weeks, until you can move into a permanent place you find.
Once you are in Munich, you should check online classified ads (e.g. kleinanzeigen.ebay.de, quoka.de, immoscout24.de) or engage a property agent (for a fee).
Also always take an experienced German-speaking friend (or the agent) along for viewings and rental negotiations. There are many pitfalls in German rental law and as a foreigner, you have no other chance of avoiding them!
The search is likely to take several weeks, and it is common to sign rental contracts 2-3 months before move-in, so you'll need the temporary accommodation for quite some time.
Good luck!

Edited to add:
Don't forget to prepare the financial side: Before move-in, you need to pay the first month rent, three months' rent as security deposit and the agent fee (another three months rent or so). In addition, you might need furniture and fittings, as most German apartments come barebones empty.
In Munich, rent for a sublet room (shared apartment) is €400-600/month and a small apartment costs €600-1000/month or more.

Thank you very much for your info. I can see now how hard it is get a rent over there...