Thanks! And excellent question. I personally bought a new phone and got a monthly plan with a German telco. But even if you're going to be there for a week, you should be able to get a prepaid sim card for about 10 Euros. These are available from lots of stores: o2, Ortel Mobile, Deutsche Telekom and many others from memory. I think Aldi actually has a particularly good sim card deal and you can purchase one from most Aldi supermarkets or their website. I would personally go for an Aldi 30 day prepaid sim card deal like this:
https://www.alditalk.de/nord/Telefonieren_Surfen/aldi_talk_paket_300.php
I'm sorry, I can't seem to find an English version of the website, but basically you get 300 minutes of calls and/or SMS in all German networks, plus 300MB of data for 7.99 per month. You can order online but you might be better off just going to an Aldi (or any other) store after you arrive. If you don't speak German, just ask for someone who speaks English. There will probably be at least one person there who does.
My personal opinion though is that you don't really need to worry about that. You should be able to make calls using Skype or similar, and there are wifi hotspots all over the city. Your hotel will probably offer free wifi and there are countless cafes that don't mind you using their internet for hours. (If you like, I can give you a list of my favourites.
)
Have a fun time!