The best resource is a knowledgeable teacher, if you have one. Don't be too embarrassed to ask them to explain something again if you didn't understand it the first time - it's more embarrassing to get a bad grade. If you don't have a good teacher, I liked ChemRevise and the CGP textbooks.
I didn't stick to this perfectly, but I recommend making flashcards on everything you cover in lesson as soon as possible. This makes sure you commit what you've learnt to memory before you lose understanding of the concepts. I think it's good to make your own flashcards, because it checks you actually understand the content and it also leads to easier memorisation in my experience.
You should prioritise understanding WHY everything happens in each mechanism you learn, instead of trying to just blindly memorise where each arrow goes. For example, with nucleophilic substitution, you should understand why an arrow goes from the Carbon-Halogen bond to the halogen (it's because the halogen is very electronegative so strongly attracts the pair of electrons in the covalent bond). This has two benefits: first, it makes the mechanism easier to memorise (I can't really prove this, but you can try it for yourself. Imagine trying to memorise a random string of 10 words vs. a 10 word long sentence - the sentence is going to be easier because understanding helps memory). Second, it means you can apply the same logic to application questions where you have to draw curly arrows on unfamiliar mechanisms, which is a question type students often struggle on.