I need to remove 100mV of ripple out of my 12Vdc source. What size and type capacitor do I need?
If you have 100mV (however you're measuring it) of high frequency ripple from a switching power supply you may be better off adding a regulator or LDO. Most regulators will let more high frequency stuff through than low frequency, so be sure to compare the regulation with an actual input from the power supply. 

For example, the ubiquitous LM317 could be used to make (say) a 10V supply, but the ripple regulation is only specified at 120Hz (-65dB typical). 

You will never be able to remove it entirely- only to attenuate it to some value. Anything you add will also contribute some noise itself. See EMF's answer for how to calculate the capacitance if you're just slapping a capacitor across the output, and select his answer if it's what you're looking for. 

Please get into the habit of specifying how you are measuring the ripple and what it looks like in the frequency domain. 100mV RMS of spikey SMPS noise might easily be 1Vp-p.