Some dogs are just not good with other dogs. Depending on how old she is, she could be like that forever. Unless dogs have experience being social when they are puppies, it is going to be hard and take a while for them to become accustomed to interacted with other dogs.
Another thing to be mindful of is your dog's previous experiences before you adopted her. Some dogs had bad experiences with other dogs and remember that forever; that could leave them terrified of dogs forever.
If you still feel it is safe to introduce her to other dogs, then do it slowly. At first, just walk them next to each other to get them used to being by one another. Let them be the ones to decide to introduce themselves, as long as they are not being aggressive. Butt sniffing is normal, and usually encouraged; it will get them more comfortable with each other.
Just remember that some dogs do not like other dogs, and some dogs do not like certain dogs. If the aggression seems unsafe for your dog or for other dogs, it may be best to keep her from other dogs.