How to create a radio frequency transmitter?

Me, myself and I, have never seen anyone use that one until now. To answer your question, the ARRL radio amateurs hand book would be a good place to start. You could ask Nuts and Volts magazine for assistance. Ramsey Electronics is a good source as well. Analog Devices Corporation make LSI type devices that could be used to make a simple transmitter as well.

Since you obviously haven't a clue about what you propose doing if you go ahead you will almost inevitably interfere with some established service. That service might be the air band, ambulance or rescue services and your interference could lead to loss of life. That is the reason why radio transmission generally needs a licence, the licence being a guarantee that the licensee knows, or should know, what they are doing. Abandon your "powerful generator" idea and stick to something simple, a single stage oscillator for example.

There are lots of alternatives. You can hook an antenna to the spark plug of a lawnmower, and that'll give you some impressive damped rf pulses. You can use a microwave oven for 500 watts of UHF at whatever frequency that is--4350MHZ? You can put a file, a battery, and the primary winding of a spark coil in series, and connect an antenna to the secondary of the coil. This will give you lots of wide-band disturbance. You can buy a used citizen's band radio at the flea market, or an amateur radio transmitter. And you can even buy a tiny multi-megahertz crystal-controlled oscillator that'll run on five volts or three volts; they use these inside computers for the clock, but you can hook an antenna to them. Beyond that, look in old amateur radio magazines or books, and they'll show you how to build any number of radio transmitters. It's really not that hard; good transmitters are more difficult to make.