How many wires are coming out of the motor? Just two? If it's just two...it really is a tiny, low voltage synchronous motor and not a BLDC motor! Weird!
In that case you have to control the motor speed by varying the frequency (not the voltage). So you need to use a transformer, bridge rectifier, and large smoothing capacitor (or just a battery) to get some DC voltage somehow.
THen you need to build a two-phase inverter circuit (an H-bridge). You then switch it on and off to make an AC-like waveform that varies in frequency. You can use MOSFETs rather than triacs since you are switching a DC voltage, and not an AC one. THe waveform you make will be more like a square wave unless you greatly increase the complexity of your circuit.
The next step-up is to add in an inductor/capacitive filter on the output of your inverter and vary the PWM input to the inverter sinusoidally. THe filter will filter out the high frqeuencies only letting the fundamental through producing a sine-wave.