How do you introduce another pet to your house

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We have a cat who is almost 2 years old. I still think of her as a kitten cause she is so petite. She is a great little cat and we all love her. Now I was thinking of getting a second one. Now what I'm wondering is if you have a female should you get another female or a male or does it matter. How can I know if they are going to get along?? Is this even a good idea or am I crazy???