Look on the web for circuits that use the LM18200T H-Bridge driver IC
Mainly proportional speed control is just an adc reading a pot, and setting a pwm relative to the position of the pot, the resulting pulses being fed to an H-Bridge driver circuit.
You can build an H-bridge from scratch, but it depends on really how much feedback you want from it, like current control etc. I would go for using an H-Bridge chip with all the trinkets already designed in and stable, and concentrate my efforts on the pwm using a PIC.
OOpic had a motor controller thingy using one of these chips if I remember correctly, so maybe you can get some information from that source ?