for this achievement you need to feed two different cows one wheat each. this will cause them to enter "love mode" ;) when two nearby animals of the same species are "in love" a baby will magically appear.
to grow wheat, you need seeds, which can be found by mining/punching grass. Once you get a few seeds, use a hoe on a piece of dirt next to water. this will create hydrated farmland and you can place the seeds there. after a growing while (about 40 minutes), it will turn brown at the tips. now you can mine/punch the wheat to harvest it.
if you equip the wheat, nearby cows (and sheep) will follow you around. use this to draw two cows together if you are having trouble finding two in the same place.