Dialogue

1. Dialogue is not like real speech. Try and transcribe a real conversation and it will be full of inconsistencies, incomplete sentences, bad grammar, interruptions, noisewords, often quite boring and be completely unreadable on the page.
But don't use it to tell the whole story.

2. Make your talkers distinguishable.
An old shop assistant doesn't have the same vocabulary as a teenage shoplifter.
A teacher talks differently to a pupil than to a parent than to the school bus driver.

3. People don't always answer the questions they were asked (look at politicians!) Evasion is interesting.
People keep secrets. Much more interesting. Leave some work for the reader to do.

4. Please, please, please don't (over)use replacement verbs for 'said.'I'm thinking here of joked (we should know from the words) replied, stuttered, exclaimed, ejaculated, yelled, stated, shouted, declared and my personal bugbear quipped.

Same goes for adverb. Use sparingly. Think Marmite. Said quietly, hopefully, tentatively, despairingly. It should be obvious from the words, the response and perhaps a bit of description of the conversationalists.

Mind you

I was still standing. He glared at me. "Sit down," he yelled

is different from "Sit down," he whispered or hissed. More menacing.

5. Also, leave out the boring bits, the how are you, the nice weather, the shall I pour your tea?.

Hi Anne. How are you?
Fine. And you?
Fine. That's a nice jacket.
Thanks
Where d'you get it?
It was a present
Nice. Who from?
Jerry
Did I miss your birthday?
No.

Hi, Anne. I love your jacket. Is it new?
No, Jerry gave it to me, out of the blue.

6. Also, try and make the speech even. So there isn't one character who does a big long lot of speech and then a one liner from someone else and another bit long lot of speech. Equally, having one line per speaker line after line can grate too.

7. Anyway, read these two articles. Here and here and again here.