I suggest you go with one of the four main GSM carriers T-Mobile, Vodafone, E-Plus or o2 and buy the card in one of their retail shops. The virtual carriers like Aldi (which uses the E-Plus network) often require you to activate the SIM card over the internet or sometimes you need a German mailing address where they ship you the SIM-Card. Being a tourist this can be tricky. If you don't have a GSM phone you often get cheap bundles of a basic GSM phone and a SIM Card.