Punctuation is kind of A Thing with me. So let’s talk dialogue!
Hard and fast rule: if you use a dialogue tag (such as “he said”), you never end the dialogue with a period/full stop. Additionally, he/she is always lowercase.
Right: “Punctuation is awesome,” he said.
Wrong: “Punctuation is awesome.” He said.
If the dialogue tag is at the beginning, follow it with a comma and capitalize the start of the quoted text.
Right: He said, “Punctuation is awesome.”
Wrong: He said, “punctuation is awesome.”
Also wrong: He said “Punctuation is awesome.”
Other sentence-ending punctuation marks (?, !) go inside the quotes with the same capitalization rules.
Right: “Does this make sense?” she asked.
Wrong: “Does this make sense”? she asked.
Also Wrong: “Does this make sense?” She asked.
If you’re doing a divided quotation, you should use commas for the dialogue tag and not capitalize the second half. This looks wrong to some people, however, and you do see the latter.
Right: “I told you,” she said, “this isn’t that hard.”
Wrong: “I told you,” she said. “This isn’t that hard.”
(Undivided: “I told you this isn’t that hard.”)
Yes, an editor will fix these things for you, but I advocate learning. It just makes everything so much easier later on.