If you're a male, you're an alumnus. If there are a bunch of guys together, you're alumni. If you're female, you're alumna. A group of females is called alumnae. However, if a group is made up of men and women, you're alumni again. That is why you see "alumni" most often; it can be gender neutral.
Are you following all of this? Alumnus, alumna, alumnae, alumni, all comes from the Latin words "alumnus" which means foster child and "alere," which means to nourish.