First and foremost congrats on the new kitten! When introducing a new pet it is always a good idea to keep them separate but within the same home and rotate out their toys and beds for a little while first so that they can become comfortable with each other's scent. Cats, know their friends by the scent that they carry. After a few days of this, you can begin the introductions. Place the new cat is his/her carrier and put him/ her in an open area (still in the carrier) the new cat will come up and sniff at him/her, give the old cats some treats and pet him/her this will help them to associate the new cat with some thing pleasurable i.e. a treat! Please be aware some hissing may be done. I recommend snapping, or tossing a pillow in the cats direction, to ease the hissing a bit. Repeat this for several days ... they will eventually get used to each others scents and presence. Remember, most cats are territorial, and your cat might feel like he/she is being replaced, so make sure you give both cats EQUAL attention. I have had this experience before and I am currently going through it again with a new kitten, so I hope this is as helpful to you as it was for me!