First off, it is far more important to know and use proper grammar than most young writers think. Submitting a piece filled with grammar mistakes is like going to a job interview in flip-flops and cut-offs. At the same time, putting stodgy dialogue in the mouths of your characters simply because it is grammatically correct will also mark you as an amateur. That applies to your characters' internal dialogue as well. Most of us don't try to remember "to whom" we were speaking the other night.
Punctuation is another tough one. Going strictly by the rules we would all use semicolons much more than we do. When was the last time you used a semicolon before "however" or "therefore?" About the only time I use a semicolon these days is Rule 4, to separate a series when one or more of the units contains a comma. Speaking of the comma, I have been accused of both over-using and under-using this little bit of punctuation at different times in my life and once regarding the same piece read by two different people.
Whether a comma should go before the last unit in a series (Meg, Susan, and I) can be arguing too fine a point. Certain stylebooks say it should, but at least one that I know of, The AP Stylebook, says it shouldn't. Only a very picky editor will deny a piece based on her disagreement with that last comma, but she may deny a piece with no comma between Meg and Susan, especially if that type of error appears more than once.
Flexibility in grammar rules will also vary depending on the genre. While I noted that "whom" often feels stilted in a short story, the sentence "He couldn't remember who he talked to about it" sounds inappropriate for a news article, unless the journalist is quoting someone.
Then there are the words misused so often the misuse becomes acceptable. I can't tell you how many times I've read that "the car collided with a pole." For objects to collide, technically both have to be in motion. And the term "comprised of" used to be considered poor English, not to mention redundent. One should say "The housing development comprises two streets with townhomes and one with single homes." Yet the former has become so common as to be acceptable.
So what is a writer to make of all this?
Before you begin submitting you should be aware of the major rules, the ones no one should break. These are rules like putting a period at the end of a declarative sentence and a question mark after a question; putting punctuation inside of quotation marks; putting the period/question mark outside parentheses unless the parenthetical phrase is a separate and complete sentence. (You will notice my use of the semicolon because "and" appeared in the first unit of the series.) You must understand subject and verb agreement, i.e., plural subject with plural verb and vice versa. An editor might let you off the hook for "The group are going" instead of "The group is going," but you won't ever get away with "They was going" unless it is the dialogue of an uneducated character. You also have to know how to format dialogue correctly.Break any of these rules and your writing will appear too sloppy to take seriously.
Beyond those rules, don't sweat the small stuff. The big stuff is knowing to place a comma before or after the tag line or start a new paragraph for each speaker.
Mary said, "I'm going with you."
"Oh no you won't," said John. "You won't be ready in time."
The small stuff, at least in this editor's mind, is something like how you would punctuate the same dialogue if you reversed the word order of the tag to "John said" in place of "said John." Is it
"Oh no you won't." John said.
"Oh no you won't," John said.
"Oh no you won't," John said.
I've seen it both ways, and to tell you the truth, I couldn't give a rat's tail which way the author decides to punctuate it. I leave that to the grammar mavens. Some people will debate the fine points of grammar like lawyers debate Supreme Court decisions. For the rest of us, it is more important to keep writing.