General advice on studying:
I recommend you read Barbara Oakley's 10 Rules of Studying.
I particularly want to emphasize two things she mentions. The first is to space out your studying. Doing 30 minutes a day of something is 10x more effective than doing 3 hours one day a week.
The second is to focus---when you're studying, only study. In particular, that means you have to get away from any people or electronic devices that might distract you. Remember: a 1 minute interruption costs you 10 minutes of productivity, because your brain is slow to shift back into gear after being interrupted.
Even as an adult, when I need to concentrate on something difficult I turn off my WiFi router and put my phone in a different room.
Read your textbook. I see many students who resist reading their more difficult textbooks (e.g. math, physics), presumably because they are hard to understand. Generally speaking, they are difficult because their subject matter is difficult, and there is no avoiding that. Better to read the textbook, become confused, and have your confusions cleared up in class than to go to class, become confused, and have your confusions cleared up never!
After a class, lecture, or tutoring session, immediately take a few minutes to go over what was said and make some notes on the key points. Yes, this is the time when you will least feel like studying, but it's also the time when the material is fresh in your mind. And after you've reviewed the lesson once, doing it a second time (ideally within 24 hours!) won't seem daunting.
If you have a procrastination problem, watch this talk by Tim Pychyl.
I think beating procrastination really boils down to two things. First, make intentions you can actually follow through on. If you form the intention "this Saturday I will study for 8 hours and then run a marathon," you will fail. Start small and don't overwhelm yourself. For example, plan to spend 10 minutes every weekday studying math (or whatever it is you need to study). Even better is if you decide the exact time you will study (e.g. "right after dinner").
The second thing: once you have formed an intention, you absolutely must follow through. Otherwise you form the habit of disregarding your own decisions, which will only make things worse! And don't bargain with yourself: even if you did extra work today, don't take tomorrow off unless that was part of your plan all along. This sort of thinking will just take you down the slippery slope of finding reasons to ignore the plan you made.
The two parts are related. Because it's so important to stick to intentions once you've chosen them, it's also very important not to make those intentions unrealistic. Say you have three hours of homework a night. If you're a terrible procrastinator who waits until 10 pm to start on homework, it wouldn't be realistic to say "from now on I'm going to do all 3 hours of homework as soon as I get home from school." You might succeed the first day, but the second day it will be too much and you'll give up. You should instead start out by deciding to do 10 minutes of homework every day when you get home from school. If you do the other 2 hours and 50 minutes at 10 pm, so be it. Once you've succeeded in that for a week, make it 15 minutes, and so on.
Once in a while, something will come up and you'll find that you've made an intention that it just doesn't make sense to follow through on. That's fine---you can cut yourself some slack every now and then. The key is not to do this too often, and to do it only for a good reason (not because you "just don't feel like it"). If you're skipping something more than 10% of the time, your intentions are too ambitious and you should scale back.
Try to maintain some awareness of how much time you're spending on things. If you seem to be spending excessive time on one particular class, you might want to re-evaluate your study methods (perhaps in consultation with your teacher). You might even want to cut back a bit (i.e. study a little less thoroughly, write less ambitious essays, etc.). I find that many students actually have no idea how much time they're spending on anything (and certainly I was guilty of that in my youth), which always makes it hard to advise them.