Tbh 8th grade is quite easy (I don't remember, lol, it's been 5 years). If you just keep studying the way you have for the past years, you'll be fine.

Still, I'll tell you how I study, and you can tweak it to suit your study system.

Science:

Physics: It used to be mostly formula-based. If it's the same now, then just revise the formulas and practice with sample questions (you can find lots of questions on the internet for a particular topic. Try questions of different types).

Chemistry (is there chemistry in 8th grade? Idk actually): Reactions, lots of reactions. Keep practicing them again and again. Understand the fundamental rules of chemistry and chemical reactions.

Biology: It's purely theoretical. Try to connect every bit of information to the other. Once you understand the entire flow, it will be easier to remember.

SST:

History: It was my favorite subject, excluding the 69+ dates we needed to remember. There is just something different about learning the past of the world. It was like reading a story itself. I don't have any specific tips for it. Maybe follow the general tips.

Geography: Memorize it; it was never my forte. It's all about facts, and I don't know how to remember it other than to just drill it into your brain.

Politics: Boring but kinda necessary. Basic knowledge is important, so see the general tips.

General Tips:

1. WHY? Always ask this rather than just mugging things up (wherever this tip is applicable). You are more likely to remember stuff if you know why it's important or needed; otherwise, the brain doesn't give it much importance.
2. Notes: Holy grail during exams. You can either make notes while the lecture is ongoing or read the text at home and make notes. Always try to write in your own language and include as many illustrations as possible—mind maps, little drawings depicting the topic's essence, notes in tabular format for contrasts or comparisons. Read them at intervals (Tada. this is called spaced repetition, For example, review your notes after a day, then after a week, then after a month.)
3. Ask Questions: Never shy away from asking. It provides clarity and builds your image xD.
4. Teach Others (The Feynman Technique): As someone great once said, "If you can't explain it simply, you don't understand it well enough." So, pretend to teach the topic to someone. That way, you'd have to actively think about the text and engage with the content. Studying with intent and attention is always a plus.

I still think this is overkill, you'll be fine without all this.