Introducing your new kitten to another cat
If you already have a cat at home, she's likely to become territorial when your new kitten arrives. It's only natural that you'll want your new pet to be accepted by your cat. But it's also natural that she might need some persuasion to accept what she may see as a rival. After all, she's always been Top Cat and ruled the roost on her terms up to now.
Slowly does it
Your two pets stand an excellent chance of getting on if you follow a couple of simple procedures. Firstly, introduce your new addition slowly. And secondly, make sure your resident cat's food and sleeping arrangements are unaffected by your new kitten. Then the chances are they'll hit it off really well. Be warned though, that there is always an outside chance that they'll never really get on at all.
When you decide the time is right to introduce them to one another, you take the lead; don't just leave it to them to get on with it. And choose a moment when the house is quiet. Since your new kitten is sexually immature, your resident cat won't feel threatened or be in competition. The risk of competition is also reduced if the two cats are of the opposite sex. But please don't rush into a face-to-face meeting; keep them apart, but let them investigate each other's living quarters and bedding. This will allow them to slowly get used to having another cat around.
A bit of common scents
Smell is the most important sense for cats, so it's a good idea to transfer some of the smells of the home onto the coat of your new kitten before the introductions. Mix the scents by stroking first your resident cat, then the kitten, without washing your hands, and vice versa. That'll introduce your kitten to the scent of your other cat as well.
Giving your kitten a place of her own
You might consider providing a pen or crate for your kitten, with her bed, litter tray and water bowl inside, so she will feel safe and comfortable. When the older cat is brought into the room to meet her, she'll feel protected by her pen or crate, yet still be able to see your other cat. And when you decide the time is right (this may take several days), open the door of the pen and let your kitten come out in her own good time.
There's no guarantee that two cats will become best friends; if they don't, let them develop whatever relationship suits them both. At the very least, most cats will learn, eventually, to tolerate each other.