Generally, I agree with Inquisitor about ALDItalk as one of the best solutions.
If you can't find any ALDI market in your area, because in the centers they are not all over, then you can go to any other supermarket chain (Lidl, Rewe, Edeka, Penny, Plus), electronic market (Media Markt, Saturn) or gas station. But try to stay away from the shops of the network providers (Telekom, Vodafone, eplus/Base, o2) as they will charge you twice.
In all the mentioned places you find starter packs of SIM cards. Here is an up to date survey in English:
http://prepaidwithdata.wikia.com/wiki/Germany For Munich it's not so important which network you are on.
Unfortunately, you can't put the SIM in your phone and just have connection. I've just registered an ALDItalk SIM for a Russian friend. Not speaking any German, he was not able to do it alone.
This I find very annoying: you will need to register your SIM first, before you have connection:
a.) either with their customer support by phone (but hey! you don't have phone connection yet) and don't know about the English skills of the support hotline.
b.) or on their website, but this is only available in German and you are not likely to have internet already. You may log on a wifi in your hotel. And with ALDI you will be guided through the whole portfolio in German. For the 500 MB pack look for "Internet-Flatrate M" and "Option buchen".
For me it's kind of crazy to give new customers only two ways of registering, but both ways require exactly the way of communication you want to establish, but can't so far, because of not being registered.
I shouldn't mention that: of course you can give any German address (or even name) you can think of, because your data will not be verified. And yes: It takes some time, until your registration is "processed" and you will have network or data. With ALDI it took me about 30 minutes, some other networks can take hours.
To ways out of the mess:
If you require an English speaking website (and can't handle Google Translate) take Lebara mobile or Lycamobile. They have good rates too.
If you require personal attention registering, go to an small cell phone "Handy" shop where they sell these or similar SIM cards, not to supermarkets or gas stations where you can't expect any support at all (not even in German).