You've written a story, you want to post it. You're not an English major, but want your story to read well. I wrote the following short checklist to help you. It contains no jargon or technical terms. Much below reflects my opinions, not rules of grammar. Remember free advise is worth everything you paid for it.
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Proofread your story by reading it out loud, slowly, to yourself. A trap for writers is they sometimes see what they meant to write, not what they actually wrote.
When reading a sentence, if it feels natural to pause, there's a good chance a comma goes there. A comma can also help the reader use your words the way you intended. EXAMPLE: He stood slowly ran to the door hesitated then opened it. SHOULD BE: He stood slowly, ran to the door, hesitated, then opened it.
Keep each sentence short. Three short sentences usually read better than one long one.
Keep paragraphs short. Paragraphs help the reader's eye not get lost. A paragraph is also a chance for the reader to mentally catch their breath.
Only use 'AND' when absolutely necessary. Too many sentences use 'AND' when using a comma or period will read better. Never have more than one 'AND' in a sentence.
Use ', AND' or ', THEN', but avoid ', AND THEN'.
Nine times out of ten, you can remove the word THAT, yet the sentence reads just a well.