With all due respects, if you don't know what's involved in increasing a transmitter's range then you have absolutely no chance of building a working FM transmitter.
Such work requires a good understanding of electronics and how components which aren't even connected to each other can influence device operation simply because of their physical proximity to each other.
If you really do want a high power FM transmitter then your only chance of success is to go out and buy one. If the law of your country prohibits this then so be it.