The simple answer is that Hulda Clark , being credited as the first to invent a Zapper, built her first Zapper to output 30,000 Hertz. The simple answer does not account for "why 30,000 Hertz?", in large part because Hulda herself has never been all that forthcoming in detailing how or why she chose to use this particular frequency. Since that first Zapper, many other models have since been invented, some by Hulda, various others by various other people, the general concept that has developed is that a frequency lower than 30,000 Hertz may produce better penetration of the beneficial effects..... try to ignore for the moment that it still remains in large part unclear as to what the exact source & cause of these beneficial effects is, but this question does seem to be narrowed down to either voltage (charge), frequency (resonance) or some combination of the two. Since you are just getting started, perhaps just stick to making a basic 30,000 Hertz unit for now and perhaps a multi-frequency capable unit for later. Either way, have fun while your are at it. In all seriosness I wish you the best and truly hope that perhaps one day you can explain to us why any particular frequency may or may not be good to use.
For just a glimpse at the less-than-simple answer, in the very first section of her first book - Cure For All Diseases, Hulda chronicles her early research of this subject matter, a path that starts out with her discovery of how various frequencies showed success at destroying various pathogens (a rather Rife-like discovery, that). She mentions how she tested then continued advancement of research along this path and came to test the use of 434,000 Hertz upon her self, and found this successfully rid specific pathogens she knew she suffered from. But then this chroniclilng path reaches a point of quickly darting and swerving about all over the map as it regards to explanations and detail, some of which appear to be clear contradictions of other findings she made within the same couple pages of this book. Along the way she makes a leap followed by the statement - the use of any frequency between 10 Hertz and 500,000 Hertz of sufficient voltage (5 to 10 volts) of sufficient duration (7 minutes) will kill all bacteria, viruses and parasites. She then progresses into mention that the use of a frequency, any frequency, may not really be all that important, but battery operation is!... and it must be "positive offset" voltage..... a veritable mystery wrapped in enigma with a healthy dose of apparent contraditions sprinkled all about..... I dunno, but this is what she has published.