Dialogue punctation
This is the single quickest way to check if a writer has any grasp of the English language. Most publishers have a computer program which examines your handling of punctuation in dialogue and auto-rejects any submissions which fail.
"I did it," she said. Note that because there is a speech tag, there is a comma before the quote marks are closed and "she" has a small s.
"I did it." She stood up. This time, "She stood up" is a seperate sentence, so there is a period after it, and a capital S on She.
Standing, she said, "I did it."
She stood up. "I did it."
Question marks, exclamation marks, periods and commas all go inside the quote marks.
Every line of dialogue, and the actions associated with it, get a seperate paragraph. You should never have a paragraph with two people speaking.
Apostrophes
Apostrophes are used to indicate contractions and possession, with possession being the one that causes problems.
It's - it is. It's late.
Its - belonging to it. The cat licked its paw.
The girl's cat - the cat belonging to a single girl.
The girls' cat - the cat belonging to several girls.
There is no apostrophe if there is no contract or possession. The girls ran.
Pronouns
Be certain that when you use a pronoun, it is clear which noun it is replacing. If you have a scene with a man and a woman, it's fine to use "he" and "she" throughout. If there is more than one man, you need to use their names, not "he". And if there is more than one woman, use names throughout.
Pronouns agree with the words they replace.
Sentence structure
A sentence consists of a subject and a verb. Possibly an object and a subordinate clause as well. But once you have picked the subject of your sentence, the rest of the sentence is about the subject. If you find yourself including a second subject, you need a second sentence.
Paragraphs
A paragraph is concerned with a single thought or action. If your heroine is getting dressed, it's fine to include all the details in one paragraph. If she is also thinking about killing her boyfriend, that gets a seperate paragraph.
Question marks and exclamation marks.
Question marks go after questions. One per question.
Exclamation marks are used exclusively in speech, one at a time, and no more than a handful per novel. Putting a forest of exclamation marks after every line your heroine speaks does not make her more witty!!!!! It's just annoying.
Commas
This is where the debate will start. Commas are used to seperate items on a list, and to clarify the posistion of subordinate clauses in a sentence. They are not used for emphasis or because you have run out of breath.
If you have so many adjectives in a sentence that you need commas between them, consider deleting the extra adjectives.
