Sampling Anti-Aliasing Filter Bandwidth
Aliasing occurs if there are signals present that are greater than half the frequency of the sampling frequency. Lower picture shows aliasing. Upper picture is OK: -



Using a filter to eradicate those frequencies is impossible because you would need a brick wall filter so you have to decide on how much attenuation you have to provide in the filter to give an acceptably low level of aliasing.

It's a low pass filter that reduces aliasing. 

So decide on the attenuation required for any aliased signals and design a filter based on (typically) multiple sections of Sallen Key filters together forming (typically) a butterworth (maximally flat) low pass filter.