Parents wont let me go out.?

Me and my friend said we would go to the park to play some soccer right after school. I'm in high school and 15 years old.I never ever went out with friends to the park or else where even though they always invited me to come. And i still don't go anywhere. I just spend my whole weekends with my little brother and parents, and playing video games. Which is pretty boring. My friends aren't gangsters and don't use drugs, in fact they are kinda nerdy and immature. I'm always so bored so i told my friend ill go to the park to practice some soccer with him this Friday after school, so then i got home and told my mom and she like flipped out saying no. Now i got in a fight with her, wich i kept telling her i would go. I'm a very nice person, with good friends and have never ever used drugs or anything else. I will admit my grades arent the best but aren't the worst( like one A, some B's, and a few C's). Why wont my parents let me go out, im frickin 15, and how do i convince them. Thank you.