Sources abound online. For punctuation, try The Punctuation Guide. Here's how to punctuate dialogue in fiction. The best way to learn grammar conventions is to read a lot. As you read, look at the way words are used and sentences are put together. The Grammarly AI can help by making suggestions to improve your grammar along the way. For story structure, check out How to Structure a Story: The Fundamentals of Narrative, or Elise Abram's The Shape of Stories: A Comprehensive Guide for Fiction Writers.