Kittens have an instinctual need to bury their "bathroom" unmentionables, which makes potty-training them much easier than a dog -- or a child, for that matter. However, you can't just plop your kitten near a litter box and expect her to get it right without a bit of coaching.
Immediately! Get started litter-training your new feline friend as soon as possible. If the kitten is one you bred at home, watch for signs of readiness such as her scratching at the carpet, which normally starts at about 4 weeks old. If you've just brought home your new bundle of furry joy, often at 6 or 8 weeks, one of the first stops on the tour of her new home should be her litter box.
Kittens function best in small spaces to begin with. Choose a room in your house that you can close off and make it just for kitty for a while. This room should include her food and water, toys, a litter box and places to hide and feel safe, such as small boxes or a bed she can safely fit under. Any room you move her to, such as the living room after you get home from work or your bedroom at night, should have another litter box. She's still little and so is her bladder, so make it as easy as possible for her to use the litter box by putting them everywhere -- a few steps gets her where she needs to go.
When you're first introducing her to a litter box, play with her a bit to make sure she's happy -- that way, she'll associate the litter box with something positive. Place her inside and watch to see if she starts scratching on her own. If not, move her front paw gently in a scratching motion to show her how to dig in the litter. Most kittens pick up on the move right away, but some take a few tries. When she's ready to get out, let her, but put her back in a few minutes later. Let her play in between tries, and it won't take long for her to figure it out. Leave the messy clumps in the litter box for a day or so to help the scent draw your kitten back in. Kittens are still babies, so be patient. Just because she uses the litter box perfectly one day doesn't mean she won't have an accident behind your couch the next day. Keeping her confined to a small room with a litter box when you aren't home will help encourage her to use it, and keeping a sharp eye on her when you are home will help you know when to run with her to the litter box -- if she ever starts sniffing a corner or pawing at the carpet, grab her right away.
Always keep her litter box far away from her food and water; if they're in the same room, place them on opposite sides. You don't want any litter stuck to her paws to make it into her water bowl. Choose a litter box big enough for your cat to turn around in when she's fully grown, but one that's easy for your kitten to climb into; if the entrance is too high, she might not make the effort. Also, pick your litter with care. Dusty litters can irritate her sensitive little nasal passages, making her avoid the litter box. There are several dust-free litter choices that will work better. Although it's helpful to leave a few clumps in the litter at first to help your kitten smell her way to her bathroom, too many clumps will turn her off. Clean the clumps out of the litter box at least every other day when you start training and every day when your kitten has it figured out. Pour out the old litter and wash the litter box with soapy water once a week, then add fresh litter.