How do I keep myself motivated to do well in school?

It’s my senior year now, and I feel as lazy as ever. I want to go out and do a ton of memorable things, but I also have to worry about my future. I’m not as smart as my older sibling, and I feel quite hopeless. It was just brought to my attention that I need a certain mark in a certain class in order for me to get into what I want for University, so I’ll most likely have to re-take that class to get a better mark =(. How do I keep myself motivated with school? I feel as if I slack off any more, then I’ll regret it in the future and it will greatly affect me. I know what I have to do, but the problem is, I can’t get myself to do it… Any helpful suggestions that can help me be more motivated or change my lazy habits?