Has your teenager been diagnosed formally with an anxiety disorder? If not, then the first step would be to take her to see your
pediatrician or her
psychiatrist. The first step towards proper treatment is proper diagnosis. There are a few different types of anxiety disorders, and they can be treated slightly differently, so this is why I recommend a formal diagnosis.
If she has been formally diagnosed, then there are excellent treatments available. Most people with an anxiety disorder will require some medication to get symptoms under control, at least initially. However, in addition to medications, other treatments for anxiety are also helpful. These could include several therapy interventions, such as cognitive behavioral therapy, which helps people to overcome the negative thought cycles that trigger anxiety attacks. Relaxation therapy can also be helpful to control the underlying baseline anxiety.
Group therapy or individual therapy is employed for both of these techniques, as well as others, and many people will benefit from a combination of group therapy and individual therapy sessions.
Start by talking to your teenager's psychiatrist or pediatrician. They will be able to assess the severity of the symptoms and make recommendations about how to proceed next.