The Latter-day Saint garment is a symbol of the covenants we make in the Temple. It also symbolizes the garments that God gave Adam and Eve in the Garden of Eden to cover their nakedness after they partook of the fruit and realized that they were naked. In a way, they're similar to the prayer shawls that Orthodox Jews wear; they symbolize our covenants with God.
Wearing garments is quite beautiful. It's nice to have a constant reminder of the covenants you've made (such as living the commandments, chastity, sacrifice, the Word of Wisdom, etc.) and how beautiful the Temple is. I love the Temple because its symbolism points towards Christ and the Atonement. The symbolism in the Temple also points towards the equality of female and male, emphasizing their uniqueness and purposes. Whoever thinks the LDS religion is chauvinist has not been through the Temple or does not understand the symbols for male and female.
The garments are only available for purchase from Church distribution centers. To purchase garments, as well as the rest of our Temple clothes (we wear white in the Temple), you have to show a current Temple recommend and a driver's license. It used to be that you didn't have to show that much ID to purchase them; it was all on the honor system. But so many people who weren't LDS or temple worthy were purchasing them and showing them to the rest of the world, so now it's slightly more difficult to purchase them. Sacred things should be kept sacred.
You start wearing garments when you go through the Temple and do your endowment (the covenants we make). It does take some getting used to in the beginning, since they raise your body temperature a little, but they are fabulous once you get used to them.