A chess champ. Even one at the age of 9 or 10 wouldn't be playing at 300 elo
If anyone is struggling at chess I can help you

I'd have tried to help you without the story anyway. Unfortunately you haven't really played enough games to really get a gauge of where you need help. So I can only really give you standard advice.
Blunder check your moves. Do puzzles to improve tactics. Try to think ahead. And learn a couple of openings.

In the midgame , Should we play Attaching or Defensive style ?
Depends on the position. And you can learn opening moves from YouTube.

I wish to get better
You have a problem with hanging pieces. Take your time and look for what is defended and what isn't. Also you seem to play without a plan. Think about your moves and what you want to achieve. Want to get your king castled? Develop and castle. Want to start an attack? Think about how you can do that. Don't just aimlessly move pieces.
im a 13yr boy that's a 4yr chess champ and the first championship of this year that's when I retired and now I'm in a chess in 7th-12th grade and 1-2 weeks I have a tournament or championship wat ever u wanna call it and I'm worst that a five year old playing chess 4 the first time that's how much I suck so I really really need help asap
This story isn't true is it?