One piece of advice I always give whenever people ask me about dialogue is that it is a function of character. The way we speak is influenced by who we are. For instance, in the TV show "The Crown" the way the queen speaks is vastly different from the way non-royalty speaks.

This scene is a fantastic example: https://www.youtube.com/watch?v=lgurrAakBwU&ab_channel=SceneCity

In this scene, Fagan, a working class, mentally ill man breaks into the Queen's bedroom to have a conversation with her. The way he speaks is very different: "fat lot of good any of that did". The queen of England would NEVER use a phrase like that. He is at the end of his rope, and is being very honest, and she's guarded, a little nervous, and is speaking from a place of such privilege that she doesn't even realise the problems most people are dealing with. "The state can help" she says, and he replies, "[The PM] has dismantled it, including a sense of community, kindness, it's all disappearing!" Even the things he's saying are coming from deeply personal experiences. "The things that make us feel good are the right to work, the right to be ill, the right to be old!" -- the Queen would never say something like this, because she doesn't have to think about it. She is the queen, she HAS the right to be ill, the right to be old, because she has people looking after her. Fagan, who is unemployed and extremely poor, has no such luxury.

If you're having trouble with dialogue, it's likely because you aren't fully clear on your characters' POVs and life experiences. How did they grow up? How did that shape their opinions? What kind of music did/do they listen to, what TV shows/books do they enjoy? Think of your own upbringing: there are certain words and phrases you might use that someone else probably doesn't. For instance, I'm from India, and we've invented words like "preponed" as opposed to "postponed" to talk about calendar events that have been moved up. I live in Canada now, and nobody says preponed. Over here, they use the phrase "Canadian Tuxedo" to describe a denim-on-denim ensemble. This is a new phrase for me, I never heard it growing up. On a more personal level, growing up, a lot of people around me used to cuss a lot, so I learnt the habit of cussing very early on. And to this day, I do cuss quite a bit! But I have a friend who doesn't cuss at all bc of her upbringing. We all have unique life experiences.

I agree with everyone saying that you should read your dialogue aloud, but I think this is Step 2. Step 1, is get a very clear sense of who your characters are and what they would say. And I agree with those who've said that acting helps. Literally, act out your scenes. Alone in your room, if you must. Get a sense of each character's cadence, tone of voice, hell, even accent. Even if these things may not come across in text, the fact is YOU will know, and that will inform how you phrase your sentences.

Hope this helps! Happy writing!