@Rimor_
I guess with grammar, make sure ur sentences have a beginning n an end. Spelling also helps. I use grammeraly regularly to make sure I'm correctly writing my story. It's a big help, n it's free too!
Punctuation is mostly about putting ur commas, periods, exclamation/questions marks in the correct place. When writing a sentence, u gotta make sure it's in the right spot or ur readers r gonna get confused. Comma's n period are ur break between sentences. If they don't know where it starts, they don't know where it ends pretty much.
U could also get someone to proofread ur work, pretty much a second opinion n someone to notice any mistakes made. It helps bc ur been looking at what you've written longer than ur proofreader. They may pick up things you've missed!
Anyway, I hope this helps!