Self motivation in school?

Hello, I'm Kathy. I'm only 13, 14 in November. But I've been very slack and lazy since I started Secondary school. My attendance is around 85%, which, in my eyes is pretty appalling. I've been acting sick, which I feel truly ashamed of. At primary school, my standards were very high, but now I just can't stop being lazy. I'm not , I'd never misbehave or be rude to a teacher. However my work is of a low standard, and my punctuality is bad. It's an ambition of mine to become a police officer, I'd love to make it into CID! To achieve that though I need to have good attendance and punctuality, good grades in most subjects. Do you have any tips to help me motivate myself before the Summer holidays end? Thank you.