What parameter of this sensor necessary to decide about the required ADC resolution?
How can I refer find such parameter(noise floor) from the datasheet of
  this accelerometer


The noise floor is suggested in the parameters in table 1 under noise performance. For instance, in the X direction the noise is typically \$150\,\text{µg}/\sqrt{\text{Hz}}\$. To make this a clearer number for your application you need to state the bandwidth of filtering you might use.

So, if you are low pass filtering the signal to a 20 Hz bandwidth the RMS noise is \$150\,\text{µg}\cdot\sqrt{20} \approx 671\text{µg}\$.

Convert this RMS value to a signal voltage seen at your ADC and you then have a noise voltage seen by the ADC.

However, you may want to read this section: 

DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF


It gives you a little more information about filtering and anti-aliasing. It's on page 11.