I see the same mistakes so often that I've started to wonder if some major word processor's autocorrect feature doesn't know the rules. The most common mistakes and their corrections are shown below. Remember, to find out how to punctuate conversation, all you have to do is grab a novel off your shelf and see how the author does it.
The most common errors - correction underneath
"Hello." He said. <-- should be all one sentence. Comma and lowercase h.
"Hello," he said.
"Who are you?" He asked. <-- all one sentence (despite the question mark)
"Who are you?" he asked.
"I'm fine," he nodded. <-- you don't nod words. New sentence.
"I'm fine." He nodded.
"My name is Rob," he said, "what's yours?" <-- spoke two sentences. Period and uppercase W
"My name is Rob," he said. "What's yours?"
"Yippee!" He shouted. <-- all one sentence, despite the exclamation mark
"Yippee!" he shouted.
Tom said, "here I am." <-- Capital to start the dialogue
"Tom said, "Here I am."
Sue laughed, "You're funny. <-- you don't laugh words. Two sentences
"Sue laughed. "You're funny."
Replacements for "said"
In general, don't be creative. Editors don't want to see she observed, she averred, she concluded, she orated, she expostulated, she acknowledged, she inquired.
The thinking is that "said" is a word that is not noticed by the reader. And that's what you want. You want the reader to stay in the story, not get distracted by the unusual or clever word used.
Asked is okay. So, perhaps, are shouted, yelled, whispered, and grumbled, but used in moderation. The difference is that they are descriptive, not just synonyms for said..
"That's funny," he chuckled. Editors wince at this (or get irritated.) You can say words, you can chuckle, but you don't chuckle words. Use "That's funny." He chuckled.or He chuckled. "That's funny." (Two sentences.)
Same with sneered, snorted, nodded. laughed, smiled: "You're a loser," he sneered. (Wrong). She laughed, "That's funny." (Wrong)
Sighed is borderline. Dictionaries allow this transitive definition of sigh: "to express with an audible exhalation". That means, "I guess so," he sighed. would be legal. But it might not be worth the risk to use it. You don't get a chance to argue with the agent or editor who reviews your submission.
There's some divergence on "hissed": "Stop sniveling," he hissed. You might get away with that. But not: "Take that back!" he hissed. If there is no "s" sound in what he said, he didn't hiss it. Best bet is not to use hiss at all.