I got a couple of A*s, As, and Bs in GCSE. I got BBCC in my AS level (that's the first year). A lot of people might call that successful results- however, I, although I am quite happy with those results, they're not a success to me. I was aiming for As, but my foot was not on the ground. I didn't put enough effort in. I didn't understand why I was studying painfully. I didn't like school very much and was ill a lot of times.
But I have changed.
The basics MUST be covered first. Get your diet (what you eat, in moderation) in control. Get your sleeping patterns in control. Get your ambitions in focus and stay one step ahead by staying on schedule, writing key events down, listening to your teachers and learning advisers. Once that's done...
Now onto the bigger stuff... Understand HOW TO STUDY - because absolutely NO ONE is going to teach you how to study, and even if they do, it all boils down to whether you learn it or not - they can't make you learn it. So understand that taking breaks is necessary, work at a comfortable amount of time per session (e.g. pomodoro technique). Make sure you have everything you need (stationary, cards, paper, A3 paper, printer, files for organisation). Now understand what spaced repetition is, and what associations and linking knowledge is, why studying at the same-ish time most days is important. Download all the powerpoints you can get for each module (and/or ask your teacher for them). Make note/bookmark/download relevant videos, past papers, mark schemes, reports, specification (all of which you can find online on the exam board sites).
The important thing is to understand why you're studying. Don't you want to learn maths (for example) so that it can help you one day or in your career - rather than studying because your parents told you to? Do you want to go to a good university, not just because it's university, but because you can learn something there?
In essence, mostly everyone can do well if they put the time and effort in. I know a lot of people may say that to you, but it's the truth. Also, I don't like the word "Study", so I use Learning from now on.