"So many things." The author scratched her head, wondering where to begin. "Here, let me give you some pointers on writing dialogue."
1. Listen to real people talking. Dialogue on the page is a lot different from dialogue you hear in real life, but listening closely will still help you. Notice the cadence (rhythm) of different people's speech. Notice how some people repeat the same words and phrases a lot. What does their word choice tell you about them?
2. Eliminate unneccessary pleasantries. When you run into your friends in real life, you probably say hi every time. You might not need to have your characters do the same. Your scenes will quickly become boring if every conversation begins the same way:
3. Don't overuse character names. "Why not, Kekla? Isn't it a good way to remind the reader who is speaking and who is being spoken to, Kekla?" Well, sure, it can be. But if your characters are constantly using each other's names in dialogue, it won't sound very real.
4. Use beats to break up long passages of dialogue. Beats include tags, like "he said" or "she asked," but they also include actions, thoughts and descriptions that writers insert to break up the dialogue. Tags and beats give the reader more information about the characters.
"Same place as usual."
"Let's go," Shawn said. He grabbed his backpack.
Eric picked up his satchel. He was ready for an adventure. "Okay. Where?"
"Same place as usual." Shawn slid his arm into one strap, and turned toward his friend.
Eric grinned. "Awesome."
Often, if you are using beats well, you don't even need a tag. (Notice, I only use "said" once in the example above.) The reader can usually follow your characters' back-and-forth for a few lines.
5. Stick with straightforward tags. Beginning writers often try to make their characters seem more interesting by jazzing up the bland "he said" tag to something like "he grumbled," or "he muttered," or "he cried." If you do it occasionally, this is okay. But make sure you really, really need it, and don't overdo it! Also, be careful not to use words that don't really mean speech, like "he laughed." It's okay for a character to laugh, choke, or gasp while speaking, but you would punctuate it differently:
"I can't believe you did that," he laughed. (NOT GOOD)
"I can't believe you did that." He laughed. (BETTER)
"I can't believe you did that," he said, laughing. (ALSO OKAY)
"What?" he gasped. (NOT GOOD)
He gasped. "What?" (BETTER)
(Think about it: you can't actually speak while gasping. Really you can only do them one right after the other.)