Losing weight

right now I'm going into 7th grade and i weight around 90 pounds, i'm about 146cm tall but i don't look that fat. i don't have any fat in my upper body but all the fat i gain goes to my legs and my butt. over the past year i've gained 20-30 pounds but i've only been eating the meals my parents cooked for me and i exercised (played with my friends for 4 hours outside after school) everyday. i don't know how i gained so much fat i don't look that fat and when i went to buy a 2-piece swimming suit at zellars i got a medium sized top and an extra large shorts so when i was gonna buy them everyone heard that the cashier said that they were too big for me and started laughing and my dad started making fun of me getting a size that's like twice the size they think i am, i almost started to cry and then i just went home without buying anything. i was so embarrassed in front of so much people i don't want to be seen in public again and i regret wasting my dad's time. i don't want anyone to laugh at me because i'm fat so i want to diet but i don't think my parents will buy any dieting pills or anything on the internet or telephone. i don't have a treadmill or any exercising equipment how should i exercise?