Good for you for getting her a friend. But you need to do some research, so please google rat quarantine and rat introductions. There is plenty to know, and plenty to go wrong.
Firstly, it is actually best to introduce adult females to adult females. Females can sometimes take advantage of new youngsters, and literally bully them. It isn't unheard that adult females will chase young ones away from food, water and shelter, which puts immense stress on the young, and can deteriorate her health. Choose an adult female, one capable of standing up for herself. When they meet, they will spend a while fighting to sort out dominance, so brawn is useful.
Check kijiji.com or craigslist.com for older rats looking for homes, as well as petfider.com for shelter rats near you.
Prior introductions, it is recommended that you have a quarantine period. These should last a minimum of 2 weeks. This is precautionary, and simply to let you determine that the new rat is healthy and does not exhibit any symptoms you may have missed when you picked her out. The sole purpose of this is to prevent infecting your current rat if the new one does have a contagious illness, which is best for both their health as well as your wallet - one vet bill is cheaper than two.
The actual introduction should take around a week. You will need a spare, temporary cage. During the introduction, you can place their cages next to one another (with some space between though), while taking them out daily to meet on a neutral surface. This gives them some basis before they meet again on the old rat's territory.
The second cage is only temporary, so it doesn't need to be fancy. Try looking for a used cage in classified sites and thrift stores. Just make sure it is large enough. Worst case senario, you can buy a large, clear rubbermaid container, cut a large chunk out of the lid, and attach chickenn wire in place. You can even just tape it from the outside, just make sure it is secure.
Also, if the rat's case is large enough, consider getting a pair of new rats rather than just one. They will settle in together much quicker when there is more than two, I have noticed from experience. Two rats will also comfort each other in a new environment, so they endure less stress. And don't worry about them "ganging up" on your current rat or anything, that simply does not happen
