Bahn.de is the official transportation page and will be able to get you from just Frankfurt to Munich (and back, if need be). I'm linking you to the English version; they have other languages available. It does look like you can't book too far in advance, at least on their website, so for a date as far ahead as August, you may want to call in order to get a quote. I imagine the people you'd speak with would be able speak English well enough.