Keep up the momentum...but first have some momentum.
If you start a semester with no motivation, you might not get any motivation until the last month of school or so. Take some classes you enjoy and want to excel in. Start studying right off the bat (try making study guides) and you'll be headed in the right direction.
Set the scene.
It's important to understand yourself and how you study best. I'm one of the (seemingly) few people that actually studies worse at a desk without music, so I study somewhere more comfortable with music I love. If you're the kind of person who needs music too, here are some of my favorite playlists to listen to while I study:
Jazz for Reading
Instrumental Beatles Covers (these two are really great for reading since they don't have lyrics!)
Also check out this playlist I made when I really needed to motivate myself to do AP Physics homework.
I study about 500x better when I have food. Fact. Keep it healthy (low-fat popcorn is my favorite!) but don't deprive yourself.
Remember you're almost done.
It may seem like you will never graduate and be eternally stuck in high school, but before you know it, you will graduate and move on with your life! Looking back, I don't regret one moment of working hard to do well in school.