How do I know where I need decoupling capacitors?
All digital ICs should have decoupling caps between their power and ground pins.  These should be ceramic and physically as close as possible to the IC.  You want to minimize the loop length from power pin to cap to ground pin thru the IC and back to the power pin.

Decoupling caps deal with short term current spikes the IC draws.  They must therefore be high frequency.  Large capacitance is not necessary, and since large capacitors usually have poor high frequency response, they are worse.  A 100&micro;F electrolytic cap is pretty useless for decoupling.  1&micro;F to 100nF ceramic is good.

As for the cap on the motor, the idea is good but I think 100nF is too large.  That could cause excessive or unnecessary current to flow in the H bridge every time it switches.  If you're only reversing motor direction occasionally, then this isn't a big deal.  If you're using the H bridge to modulate the apparent motor drive with PWM, then you should lower the cap.  Something like 1nF should still cut down the noise the motor is making while not getting in the way of switching.