How is my acne caused?

I recently have gotten acne towards the end of my 7th grade year. Not TONS. But a moderate amount. I get them on my forehead and my chin mostly. I don't understand I have always washed my face, and now i do it like 3 times a day! My skin isn't dirty at all so how come i keep getting zits. I don't pick at them? I don't know. What is causing my break outs? (btw I am not stressed)