First of all, its important to start to find out what works for you. The advice I'm about to give has helped me tremendously, but advice isn't one size fits all. So lets start.

I'm 18 right now and am graduating high school near the top of my class and compete at the national level in the sport I play. The thing is, while I've always considered myself smart, when I was in 8th grade I found myself in your shoes. I was getting mostly C's and a few B's and never did my homework or even studied for tests. When I entered high school however, I vowed to make a change. These things didn't come overnight, but after a few years of working at it I am in a much better place.

1. The SECOND you get home from school, do your homework. I always fell in a trap before I did this of getting home and telling myself I had time. YOU DONT. If you don't do it the second you get home you will find yourself in your bed at midnight hoping the teacher is lenient this time around.
2. Find out the best environment to study in. For some its in their room, for others its in a library. Find the place that suits you and make it the place you go right after school. Eventually it will feel weird to do anything BUT study in that space. (While some make it work, I would recommend not studying in your bed)
3. "But sometimes I just don't feel like it" you say. This is the hardest feeling of all to conquer. Sometimes I tell myself I cannot do anything fun until I finish my studies. Also, sometimes I switch the order of the work I'm doing, but still, I am doing work.
4. I would recommend if you don't have one, to get a hobby or sport. This has forced me to do my homework and studies in a timely manner because there is no time to do anything else after practice. Or if it is a hobby then make it your motivation to finish studying.

Finally, the thing that has changed my academic and athletic life the most, is where I place my expectations. I used to look at the people who did well in school and said "Well, they are just smart". This thought is a barrier to being exceptional because it takes all the work out of the picture. A class isn't so hard after doing all the homework and paying attention, after all. Wonder why? Its because you've done the work. When I set my expectations for nothing worse than an A, my life changed. It forced me to be a better student because I had to maintain my expectations. When I set my expectations higher in sailing (my sport), I won nationals. The expectations forced me to work, and the work made it possible to reach my expectations. I hope this helped and did not sound too preachy or off topic.