The genus Ananas is in the family Bromeliaceae of which Bromeliads come. Bromeliads are monocots that usually have long slender and basally arranged leaves. They are known to have many specialized features such as water/nutrient storage structures between leaves that collect falling water and debris to aid plant growth. Many bromeliads are epiphytic and grow high above the ground attached to tree branches. This is not the case in Pineapples which are instead terrestrial, favoring the formation of strong roots that anchor themselves to the soil while collecting nutrients and water.
Propagation of pineapples can be done through three reliable methods, by seed, by top rooting and by vegetative "pups". Propagation by seed and top rooting are the only options if you don't already have a growing pineapple. Firstly you must obtain a pineapple, which are common almost year round in many U.S. grocery stores. It is best to choose pineapples that have healthy looking tops, especially if you choose to propagate by top cutting. Avoid pineapples with browned or yellow top leaves.
To propagate by seeds you must cut your pineapple and examine the flesh for small reddish-brown seeds. The seeds are only found in the outer flesh of the pineapple in small compartments under the hardened "skin" of the fruit. I've found that some pineapples have very few seeds or even deformed seeds which is likely the product of industrial cultivation methods. If you can find some seeds, collect them and wash them. You can plant the seeds in potting soil or wrap them in a wet paper towel and place them in a plastic bag. Be sure to keep the soil or paper towel moist and in a warm place. Within a month they should sprout. Pineapple seedlings are delicate and require warm (70-80F) temperatures and high humidity.
To propagate by pineapple tops (probably the easiest method), you must gently twist the pineapple top at the base where it connects to the fruit. It should twist off with a little elbow grease and you will be left with what looks like a young pineapple plant. Remove the remaining pineapple flesh at the bottom of the fruit to prevent rot. Next remove the small leaves at the base of the pineapple so you expose 1/2 inch to 1 inch of stalk. You will see small roots and root buds, with proper care these will become vigorous roots and your pineapple top should become a healthy pineapple plant. There are many methods for encouraging these roots to grow, whichever you choose you should place the plant in a warm area away from direct sunlight. You can wrap the exposed roots in a wet paper towel and place a plastic bag or saran wrap around the bottom to prevent the towel from drying out. Alternatively you can fill a glass with water and place the top in the glass so that the roots are submerged. The water should be replaced every few days for better results. In less than a month, if you are successful your pineapple top will have healthy young roots and it can then be transplanted to soil. If you want to avoid all of this hassle you can plant the pineapple top directly in soil, which has a lower success rate.
After a about 2-4 weeks the pineapple should have vigorous roots, as pictured below. Sometimes, however bacteria and fungi get the better of a pineapple crown, resulting in rot. Be patient and plan on experimenting with a few pineapple crowns to ensure success.