Many girls become less physically active once they become teenagers.  Here are some of the advantages of staying physically active according to the Better Health website:

• Increased strength, stamina and flexibility
• Helps maintain a healthy body weight 
• Improves fitness and heart health 
• Decreased incidence of stress and depression
• Increased self-esteem
• Positive body image
• Opportunities to have fun and interact with friends and family.

You don't have to go to the gym - there are thousands of ways to get moving:

• Go for a walk/run
  
• Walk the dog
  
• Go for a swim at the beach
• Ride your bike
• Try Pilates or Yoga if working up a sweat isn't your thing 
• Get some friends together and hire a tennis court and have a hit
• There are thousands of workouts on Youtube
• Go skiing (ok, that depends where you live)
• Go ice skating
• Go rollerblading
• Enrol in a dance class
• Your local community centre has lots of well priced exercise options.

As the prophet Nike said, just do it!