What can I take over-the-counter to calm my nerves or stop my anxiety?

Exercise. Camomille tea, kava kava, valerian root, st. John's wort. Camomille tea is the mildest. These are all herbs and herbs may be potent with potent side effects. A better way to calm anxiety is exercise. Go outside and take a long walk. Take deep breathes, breathing out slowly. Practice yoga, meditation, or mindfulness.
Drugs are not best. The most effective treatment for anxiety is relaxation training coupled with cognitive therapy that helps you modify the thoughts, perceptions and beliefs that trigger the anxiety. You can learn these skills on your own, but having a mental health professional "coach" you is quicker and more effective. Biofeedback helps (www.Bcia.Org, www.Aapb.Org). Entrenched anxiety may require meds.
Non medical treatmen. There are many things you can do to feel more calm. 1. regular aerobic exercise 2. healthy food intake and nutrition 3. adequate sleep and rest 4. meditation 5. regular relaxation / activities to get your mind off bothersome subjects.
Exercise!! Aerobic exercise is the best medicine. When anxious your body produces a lot of stress hormones such as cortisol and physical aerobic exercise helps burn that cortisol and release endorphins (calming hormones). Aromatherapy is also helpful: "Modern Essentials: A Contemporary Guide to the Therapeutic Use of Essential Oils." Mendell's Vitamin Bible (Food to increase Tryptophan).