try and find a PAIR of cats that are already bonded, 4-9 months old at least. Definitely get kitties that have been socialized with other cats (not separated from siblings at 6 weeks) best to get kittens 12-16 weeks old if you MUST have a kitten.
If you can't get a pair who are already bonded, it is believed that you will have better luck with one of each sex. good luck and post pictures.