I used to read comic books as a kid. I would save up my allowance and walk two blocks to the local drug store and spend my money on an assortment of superhero comic books. The racks were full of possible adventures for only 25 cents each. (Yes, I am aware that price point identifies me as an old fogey.)
I of course had my personal favorites, but even at 10-12 years old, I noticed that not all stories were created equally. One thing that stuck out was poorly written dialog. At ten years old, I couldn’t have correctly analyzed what was on the page and stated why it was wrong, I just knew that it didn’t sound right to my mental ear. Occasionally I would have to get help with a word; at that point my mother would make fun of the dialog in question, further cementing my impression that something wasn’t quite right in the speech balloons.
Not everyone has a natural ear for dialog, just like not everyone has an eye for describing a setting (my weakness as a writer). The best way to learn to write dialog is to sit in a public area and listen to how people interact with each other. Don’t worry getting all the words as much as accurately capturing the tone of the speaker. We speak differently to our parents than we do our neighbors, or the kid at the coffee shop counter. Police officers speak differently on duty than off, and that difference is magnified if you listen to a police scanner during a tense situation. Listening to people interact will go a long way in assisting the storyteller with writing dialog.
One last thing: I strongly recommend using “said” for all speech in written prose, e.g. “he said.” Occasionally, you will need to have a character “reply” within the confines of a conversation or perhaps “mutter” if the character has to speak in a low voice. People “say” things, so we should use “said” with our dialog.