I'm really happy I got a second cat. I bought my second kitten when my first cat was still a kitten. It was difficult for them both at first and it's eight months later and they get on really well.
This site has an article about choosing a second cat.
Once you get the new kitty you need to introduce them slowly. You put your new kitty in a room with all the things it needs and put a blanket in with it. The idea is to get the kitten's scent on it. You could also rub it on your kitten's cheeks. Then you give it to the resident cat.
Your resident cat will be able to hear your new kitten for the week or so that she is in the isolated room. They may even be on each side of the door. The next thing is to place the resident cat in the isolated room for a few hours while the new cat explores another room where the cat usually stays. This gives them the chance of getting to know each other through their scent. The next thing is to have them see each other through a crack in the door. And once they feel comfortable you introduce them physically. You play an interactive game with them. You don't leave them in the same room alone until you are confident that they won't hurt each other.