memememememe!!

You might want to start by calculating you boats resistance or the
estimated power required for a particular speed first.
Use can this calculator www.psychosnail.com/boatspeedcalculator.aspx

This will give you the SHP, shaft horsepower for a desired speed.
Since you have an outboard? it's likely that the manufacturer will be
able to tell you what diameter is best for the engine. So what's left
is calculating the pitch which isn't all that difficult, once you have
the results from the speed calculator.
Because your boat would be relatively fast the Crouch slip method
should give reasonably accurate results.
Slip is calculated as follows:
Use about 90% SHP/RPM and the speed from the BoatSpeedCalculator
P = (kts*1215.6) / (RPM* (1- (1.4* (kts^0.57) ) )

RPM is at the shaft so you need to find your reduction ratio

The inputs you'll need for the calculator are
Displacement, Length at waterline, desired speed
and the to calculate pitch RPM (with reduction)

Hope this helps