I am not sure what you mean by "black and white"? Two colors only?
If you want grayscale from color, you can type:
convert <input> -colorspace Gray <output>
convert <input> -type Grayscale <output>
These will convert color to gray.
However, if you want two colors only (black and white), then you need to threshold, for example, to select the color where above will be white and below will be black
convert <input> -threshold xx% <output>
where xx is in range 0-100 (for percent)