You could use a
Picaxe, programmed in the BASIC language, which is the easiest computer language to start with and learn for small projects.
Many on these forums use the Arduino but that uses a variant of the C language which, to me, has an arcane notation that is much more difficult to learn.
You likely will have to add some loop compensation in the control program to achieve a stable generator output voltage (avoid hunting or oscillation in the speed) which will require some form of
PID or
Fuzzy Logic (which I think is easier to use with a micro)
.