First off: I don't think anyone is an idiot, high or low grades. For various reasons I think so, which I won't go into right now. The important thing right now is finding a way for you to study ^^
What helps me:
Writing a schedule.
An ELABORATE schedule. What do I do every time of the day (when studying).
1) Basically I organise it all: I write down what subjects I need to study for each day, I do this by finding out the deadlines for the homework or tests. And which subjects I have more difficulty with than others (those I spend some more time on).
2) I wrote down how much time I THINK I need for each subject. See, especially in the beginning, or when new to writing schedules, you need to kind of figure out for yourself how much time you usually spend on a subject. So maybe you estimate to little time to study for something, or too much time. You then adjust said amount of time next time you write a schedule. You can do this by seeing how many pages or chapters you go through/work through in... 15 minutes, half an hour, even an hour. It's basically a try-out-and-see-what-works situation.
3) Write down time of the day and length I study for the subject. Example: English 13:30 - 14:00 (30 minutes). Biology 14:00-14:45 (45 minutes). Etc.
4) Make sure to add in breaks. Depending how long you can study for, and how much you keep in your noggin' x3 But in general, what worked for me: 15 minutes as smaller breaks after an hour of studying, or two hours if I'm on a roll. 30 minutes as a longer break. As of which: The breaks MUST be distracting. You can draw, watch a video, listen to music. Anything else but studying (unless it's random fun stuff you like to learn about).
5) Don't work until deep in the night, UNLESS it really, really works for you. But your body and mind need rest. Sleep, but also just distraction from studying, otherwise you keep your mind constantly occupied with studying and might actually be hard for you to learn new things.
Other tips:
Go to bed on time, and get up early.
What also can help: Studying with someone, preferably also following a schedule. Or once you've learned a subject and want to test yourself, either ask someone to ask you questions about said subject or make your own fun little tests ^^
Just make the schedule at least one day in advance, or maybe write a schedule down for a whole week. That way you don't have to stress each day about making one for the next day and just need to do it once a week.
It can be tedious and take a lot of time, but trust me, it can be worth it. And might actually help you not only work, but maybe even faster than without one.