If you weren't lucky enough to have a classy mother and you are ignorant to what, when, where, why a slip, please get yourself to a local lingerie shop or Macy's.
Don't ask your coworker or goofy roommate if you NEED a slip. Don't just stand in front of your bedroom mirror, twirl, and nod approvingly at yourself. Do one of two things:
1. Start wearing a slip with any and all dresses and skirts today. Accept more compliments than you've ever received regarding how good you look.
2. Spend a week really studying women you see who are wearing dresses and skirts, then decide if you can tell if she's wearing a slip or might look better with one. Then buy yourself a slip or two.
Yesterday I saw a very heavy woman wearing a long dark-colored maxi dress yesterday with a little capped-sleeve jacket. I am sure she felt covered up and appropriate. She was walking in the sunlight up a slight incline and the skirt was clinging to her belly and going in between her thick legs, A slip would have prevented the clinging belly fabric and would have flowed easily as she walked. Had she been wearing a slip, you wouldn't be reading about her right now.
I also observed a skinny girl wearing a thin-fabric dress and the dress clung awkwardly around her legs and butt. Very distracting. A damn shame. A slip would have made her look PERFECT.