4 Answers

   Exercise everyday watch what you eat and eat senceable

Sweetie, you haven't tried everything, and I hope you never do.  The general guideline for losing weight is to eat less and get more exercise. 
Can you get involved in a sport of some sort at school or through your city?  Even going for a walk or bike ride is good exercise....making sure you are not alone.
It there is a gym nearby and the cost is reasonable, that would be another option for more activity.  Swimming is good; if the gym has a pool, swim all year long!
Talk to your folks about a weight loss program or getting to the doctor for some advice (NOT PILLS OR JUNK LIKE THAT) and guidelines for your age and wellness factor.  The doctor may want to get a blood test done to make sure all your organs are functioning right (thyroid problems can trigger weight issues).
Girls tend to be plump, especially when they start developing breasts and their periods begin.  My best friend was awfully chubby right through 8th grade.  After that summer break, we entered high school and she had lost 20 pounds.....just her body changing from adolescent to teenager I guess.
Pimples are a pain, and probably age and development related, too.  Keep your face clean, using an acne cleanser.  Try to avoid wearing makeup, especially oil-based foundation.  If you can get a product like Proactive and use it, you should find it very helpful.  If you are facing a real acne problem (deep pimples and blackheads), ask your parents to take you to the doctor.  Your school nurse might also be a helpful resource.
Please don't let your weight and complexion keep you from being active and involved in school and community.  Keep your head up, do your best in school, and SMILE.  
Let me know how you are doing.   

How far do you walk everyday?  That's a good starting place and read the answers from Bob/PKB and Flip as well.

Are you overweight, or do you just think you are? How much exercise do you do each day? What types of food do you eat? Kids your age often have skin problems that get better as you age, but talk with your mom or some older adult what you can do for a healthier skin. Junk food often causes acne problems as well as weight, so try to stay away from that and eat healthy.