THere's more theory behind the speed control of an induction motor than any other motor I've come by in class...if you didn't already learn the circuit you didn't already take the right class which also has everything else you need to know in it too (and there's way way more theory than the circuit).
That said, you first need a 3-phase bridge rectifier with filtering to convert AC into DC (a very very large 3-phase AC to DC converter).
Then you need a 3-phase inverter that will take this DC power and use PWM to produce a sinusoid output that can vary in magnitude and frequency. PWM makes square waves only, so you must modulate the PWM duty cycle sinusoidally, and choose a frequency high enough that the square wave harmonics will be filtered out by the motor to it only sees the sinusoidal fundamental frequency waveform, or you must add filters of very high power on your output circuit to do the filtering if you cannot get PWM frequency is not high enough.
Just my opinion...but for a project like this, with such complexity and high power levels, if you are asking where to start, then you do not know enough to finish the project. At the very least, reduce your power level to 200W. Many, many problems arise in high power motor drives which can normally be ignored in lower power drives.
Attached are the two circuits you need to build and connect together (without the control circuitry or many of the inverter components needed in real life like flyback diodes and capacitors to make it work). If you weren't sure, the switches for the inverter are supposed to transistors. If you couldn't figure that out then you need to switch projects right away.