A second kitty is a great idea! I did the same thing for my cat, Stormy (also a female). I knew she got lonely during the day.
There are several ways to go about getting another cat. You really could get a male or a female. I got a male and it has worked out well. Females can be pretty territorial, so a male-female combination is good. Just make sure he is neutered and she is spayed!
Another thing to think about is age. How old is your cat? The younger she is, the better she will adapt to a new cat. Stormy was a year and a half old. I had wanted to get a kitten, because I had heard that sometimes the old cat will "mother" the new kitten and accept it well. But I ended up getting another kitty that was a year and a half old. I probably would not match a fairly old cat with a kitten because a kitten will probably bug the crap out of it, always wanting to play. Ideally your two cats should have somewhat similar energy levels. Another thing to consider is that there are so many adult cats who need homes...kittens are easy to place but there are TONS of adult cats out there who are absolutely wonderful.
Another thing is personality. If your cat is very laid back (especially with change and other animals) this will probably be easy. If not, you may have to be more careful. In my case, I knew Stormy could be pissy about other animals, so I got Justin who is a very laid back male. She is the dominant one in the house.
As for introducation...slowly is the key! You don't want to **** Marla off to where she will never want anything to do with the new cat. Confine the new cat to a bathroom or other small room when you get him. He will get used to your house slowly this way for one thing. You go in and play with him, spend time with him to get him used to you. Meanwhile you are lavishing attention on Marla! She knows something is going on, she can smell this other cat. But if you give her a lot of attention, treats, playtime, she can associate this change with something good! Continue all this for about a week. She will smell this other cat through the crack in the doorway and if there is enough room may even flick paws underneath the door at each other. Don't worry if she hisses at the doorway at first, this is normal.
Next step is to put your new cat into a carrier and put the carrier a few feet outside of the room he was staying in. Allow Marla to come over and investigate. She will be able to see the new cat but cannot get to him. She may still be hissing some at this point but hopefully this will get better over time. Continue this step for a few days or until you feel comfortable actually letting them see each other unrestrained by a carrier. Monitor their time together and just watch how they react.
It took some time for Stormy and Justin to love each other...maybe a month total or a little more. But it worked! Justin was never a problem but Stormy was pretty pissy about the whole thing at first but she got over it. The hissing subsided and now she grooms him, they play with each other and sometimes sleep together.
Again, so much of this depends on your cat. But the main thing is just to give her a lot of attention, and build up her curiosity about this new cat.
Whew, this ended up being really long but I hope I answered your questions! Let us know how it goes if you get another cat!
Edit: If you are worried that they might have trouble getting along, you might try to get a cat from someone who is fostering on the condition that you will return the cat if the relationship does not work out. This is what I did...I knew if things didn't work out, I could return Justin to the woman I got him from. Of course, that would have been tough because I had already fallen in love with him!
