There are many white papers on here about these sorts of subjects. I think your best bet will be to go with one of the gearboxes already on here. I can speak from experience when I say that designing a tranny from scratch is pretty darn hard. I just tried modifying one and managed to screw it up.
If you're trying to make one yourself, though, Martin Sprockets has a pretty good guide to sizing gears in their eCatalog. Also, I recommend Mark's Standard Handbook for Mech E's, except it's rather expensive and filled with stuff you don't need. Machinery's handbook could also be helpful. I'm unsure there is anything that specific as to tell you exactly how to design a tranny, though.
Finally this white paper seems like it would be the most helpful:
JVN's FIRST Mechanical Design Calculator