SOOOO this is 8 years ago but i just spent ages figuring it out anddd - A mimima is the point of 0 displacement, so like on a sin wave a minima would be at 0, 180 and 360 degrees- A maxima however is the point of maximum displacement, so using the sin wave again itd be the points at 90 and 270 degrees, (just cos its "down" doesnt mean its a minima...)
Think of it in terms of magnitude, bigger magnitude is maxima, smaller magnitude is minima