UPS just delivered my big box of diapers! Hooray! They're already in the wash getting prepped! So when you get new diapers, you can't just put them straight onto your baby's butt. A certain amount of preparation is required. For your regular old prefolds, you should wash them three times in hot water with a little bit of your diaper-safe detergent. Dry them in the drier between each wash. Test them before you start using them by pouring a little water on one of them. If it absorbs, you're good to go! If the water beads, try washing them again, they may not be ready. It's the natural oils that prevent them from being absorbent immediately.
Most of all, read the instructions that come with your diapers. They might have something special and different to say about them. Okay, time to go throw my fluffy mail into the drier. :)