If your cat seems to prefer being the only cat, then you should respect that and not try to change your cats preference. When selecting a new cat, try to find one that has lived with cats before. It is best to introduce a cat that is different in age and sex to your resident cat. Introducing a kitten is ideal because it will be the least threatening to your cat. However, if your cat is a senior citizen, spare it the nuisance of a rambunctious kitten and get an older, mellow adult instead.
Cats are less likely to fight when on unfamiliar territory.
So bring your new cat home in a carrier and keep him confined to a single room for a few days. This will allow the new cat a chance to become familiar with his new territory. It is important that he feels secure in his new territory before meeting your resident cat. Begin the introduction by letting the two cats get used to the smell of each other. Bring some of the resident cats bedding into the new cat's room.