1) Dialogue in fiction is nothing like how people talk in real life. It’s fine to use "as heard in real-life" phrasing, but real-life dialogue is often meaningless. Every single word spoken in fiction must be dripping with meaning. If it has more than one meaning (subtext), all the better.
2) Start the conversation late and exit early. No one wants to read small talk, hellos, or goodbyes unless they add meaning to the story … which is almost never.
3) Dialogue should always progress the plot or character development. No info dumping in dialogue, please. Only put quotation marks around what you can actually envision the character saying in that particular scene, knowing and feeling what that particular character knows and feels. What you want your readers to know has nothing to do with what a character actually says. After all, that's why stories have narration.
4) Be unexpected and don’t fill dialogue with cliches. If readers can predict what the next character will say before they say it, you need to spice things up or revisit why you’re writing that particular bit of dialogue into the story in the first place.
5) Give characters unique voices. Just like people in real-life develop different speech patterns based on where they're from, their education, work environment, and even religious leanings, fictional characters should have the same rich and unique voices. If you can't tell which character is speaking just based on the way they string a sentence together, stop and take a while to consider how the characters would speak differently and why. (The only time it's OK and maybe even expected for two characters to sound similar is if they are family members.)
6) Don't overwrite dialogue. This is one of the biggest mistakes I see novice writers make. Keep it short, and don’t give characters line after line of redundant or overly wordy dialogue (unless it’s a deliberate character flaw that’s obvious to readers). Figure out exactly what your character is trying to say and the emotion behind it, and say that as succinctly as possible. This will give your dialogue greater impact, and it won't waste the reader's time.
7) For the love of all that's shiny and chrome, learn how to punctuate dialogue correctly. Remember to use action tags instead of lengthy dialogue tags, and please, I'm begging you here, start a new paragraph every time a different character starts talking. This is basic, basic stuff. If you need more education on the subject, crack open a junior high English book or check out this article and the dozens of others like it that can be found in 0.38 seconds by an amazing search engine colloquially known as Google. There is no excuse for bad punctation, and everyone will judge you for it. I say this not to be mean or rude but to keep it real. (Honestly, I hate that I have to say this at all.)
You can do it! Let your inner characters free and go write awesome dialogue!