@TMaxKnow your character inside and out. Write it the way that character would say it. If it's a stuffy professor, write the dialogue in formal English. Maybe use big words if he is the kind who wants to impress people with his knowledge. If it's a tough, uneducated, streetwise kid, write the way that character would speak. If a character uses a lot of slang, well, have him talk with slang. Or maybe a character speaks in cliches. If a character lacks confidence, put in pauses.
The dialogue can give the character, well, character. In "The Last Boy Scout," one of the characters always called people by their formal name even if they didn't go by it. So he would call the main character Joe, Joseph. That had an impact on me. I remember it to this day.
Other than that stuffy professor, people don't speak in complete sentences. But don't go overboard with that, just like don't go overboard with pauses (e.g., um, uh). You still want it readable.
I'm sure if you Google "How to write dialogue in fiction" you'll get a lot of opinions, some good, some not so good. Choose the ones that help you.