Your radio is probably VHF or UHF, and 50km with any kind of reliability is not a realistic goal for a portable radio at those frequencies (Apart from anything else you pretty much need line of sight between the aerials), also VHF & UHF amplifiers are in no way beginners projects, certainally not at the power level you would need.
The usual way to make handheld radios work over greater distances is to place repeater stations in high places so the handheld radio only needs to connect to the repeater, but that technology gives even seasoned engineers headaches.
Sorry, but this is not a beginners project.
Regards, Dan.