I should say that that depends on the typical wind speeds you have, on the design characteristics off the wind mill airfoils (drag and lift), and on the generator minimum and maximum shaft speeds and resistant torque... It is indeed a complex design theme...
If you are planning to use a gearbox, you should keep in mind that the most efficient gearbox are those who use straight-cut gears (spur gears)... In general, The amount of gears you need will depend on the in-out ratio, on the torque, on the speed, on the size limits of the gearbox, and on the gear materials... Then you will also have to think on the shafts, on the bearings and on the lubrication... Usually gearboxes are an expensive solution.. (Here is a nice engineering software that can help on some of these gearbox related issues
http://www.kisssoft.ch/english/products/kisssoft.php)
Here is a nice article:
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A simpler solution would be to use a simple belt transmission... Belt transmission can also achieve a pretty good efficiency, good in/out rates, and are simpler to design ( you can usually do it just by following the catalog selection instructions), and of course, much more cheap...
Here is an example of a detailed selection helper catalog **broken link removed**
About the gears.. If you want to know more about how to design a gear box, you can try follow the instructions specified by the normative reference
ISO 6336:2006
A little more info on wind turbine design can be found on
IEC 61400:2012 (described in
http://en.wikipedia.org/wiki/IEC_61400) with focus to part 4 -
IEC 61400-4 Design requirements for wind turbine gearboxes
Anyway, don't forget that after this effort you will probably need someone to make the gears that you calculated...
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Forgot to mention that bearings can easy be selected from a catalog or one online selection tool from manufactures as for example FAG and NSK:
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You should also check if the shaft can handle the torque, bending and fatigue, and if there is any resonant problem