Worried about failing high school, what do?

I live in Sweden and I recently started high school (junior year with the American system). What I chose to study was Natural Sciences, which focuses on Physics, Math, Chemistry and all that fancy stuff. This is pretty much the hardest thing you can study at high school level in Sweden, and I entered this with a bit too much self-confidence. I'm only about 3 months in and I'm already failing 3 classes and I'm getting mediocre grades in the rest.

I've always been a lazy piece of shit that procrastinates too much and I spent the last year of compulsory school by sleeping through basically every class, but I still managed to get above-average grades to be able to apply to the school I go to now. Now that I'm in high school, my laziness is coming back to bite me in the ass. I'm aspiring to become a veterinary or something in the Biology profession but that will never happen if I keep on like I do now. The worst part is that I barely care about school and never learn from my mistakes and even if I get a failing grade on a math test, I won't study harder for the next one due to said lazyness.

TL;DR: How do I stop being a lazy shit and start studying more?