Congratulations on your new gerbil! Gerbils are social creatures and it's great that you want to get her a friend! However, gerbils are also highly territorial so they are tricky to introduce. Females are even more territorial than males, and can fight easily. The correct way of introducing 2 gerbils is the split cage method. What you have to do is get a divider to split the tank in two. This divider should be strong and sturdy enough so that the gerbils can't pass to the other side, yet it has to have holes so that they can see, smell and, to some extent, touch each other. The holes must not be big enough so that the gerbils can grab the other through the division and hurt it, so the best thing to go is with a screen divider. I've heard that many people use a screen lid turned to it's side, but I haven't tried it. What I have is a divider I build with wood and screen mesh that fits just a tiny bit bigger than the tank so it get's stuck and the gerbils can't move it. Here are pictures of my divided tank:
The divider is removable, so I only put it in when I want to introduce 2 gerbs or if I want to keep 2 pairs separated in the same tank. This tank is a 20g long, but you can do the introduction in a ten gallon too.
After you have 1 gerbil in each side, you have to switch them once or twice a day, so their smells mix and they get used to each other smell. Be careful when doing so, use one hand for each gerbil and don't change, or they can bite you cause you have the smell of another gerbil on you. With females, you'll prbably will have to do this for a month or so, as they are a bit stubborn. You'll see signs of friendship when you see them sleeping by each other near the division, trying to groom each other through the division and stop attacking the division. It will be easier for you to introduce a 6 week old pup to an adult, as pups are less territorial (cause they are not sexually mature yet) and adults tend to be less territorial around them, but still do the the split cage!
When you think it's time to put them together, do so in a neutral territory (like an empty cage or bin that none of them have been before, or a bath tub). Wear gloves, in case you have to separate a fight (I've only been bitten by gerbils twice and both times it has been separating fights, the sweetest gerbils, but they are just doing what their instinct tells them to do). I've also read around that a dab of vanilla on their noses and bums helps disguise the smells and makes the introduction easier, I've never tried it, though.
Even though this seems like a lot of work, your gerbil will thank you in the future for getting her a friend. Lonely gerbils tend to get depressed and in general live a shorter life than a gerbil that has a cage mate. Good luck with your new girl!
PS. what's your gerbil's name? Do you have any pictures? 8)