I need some help. I'm considering for some time a monosylabic conlang and I
think tones would be very suitable for it, but...

Tones are a very exotic thing for me and I have rather vague idea of
uttering them and almost no idea of their possible origin. How can tones
come into being? Are there any general rules or tendencies? I assume that
monosyllabic morphemes of my language are results of reduction of
polisyllabic words. How can such a reduction affect tones?

Are there any tonal languages with well known tonogenesis or evolution of
tones? I expect Tibetan to be quite interesting, but I don't have any source
on it and I havn't found anything in the net.

Łukasz K.