Something about this feels... off.
Have you ever thought this to yourself when writing a conversation between characters? Maybe you even considered it when you were writing a segment of inner thought? If not, Congratulations or I'm so sorry. You might be asking, "Aaron, what do you mean?". What I mean is one of two things: You don't have this issue, OR you have this issue and don't see it because you're too attached to the piece you're working on. I have experienced both, so I'll try to help you out the best I can. This is a basic overview of the complexities that dialogue holds.
What do you mean "my dialogue sucks"?
It was a bright sunny day. My college classes had ended for the day, I didn't have any homework, and the ROTC program had been suspiciously quiet. My wife (girlfriend at the time) was working, and I had the entire day to myself. For once, things were calm, and everything seemed harmonious. I could finally write in my book.
Back when my book was actually terrible (it took me a long time to see that), I still loved to send it to my family. They were, after all, my fan base and support system. I could always count on them telling me how much they enjoyed my overwritten chapters and that they couldn't wait to get their hands on another one. That is until my brother finally spoke up.
I vaguely remember how he put it, but it went a little something like *ahem* this: "You're dialogue sucks."
Taken aback, confused, and slightly offended all at once, I proceeded to ask "Whatever do you mean?" (Most likely not like this, but it's for show, so :P ).
He then took it upon himself to bluntly explain (something my brother has always been good at). Turns out, my lines were cheesy, corny, awkward, and downright cringe. You can probably imagine how receiving that news went. However, after a solid day of reflection, I realized that he was absolutely right. My dialogue truly and utterly sucked.
That leads me to my first piece of advice.
Did someone say "All-natural"?
I can't stress this enough, Ladies and Gentlemen: Dialogue MUST be natural.
Even if you have the world's most awkward character, the dialogue needs to feel authentic. That means you needed to have established their inner voice at the very start of their first scene. The dialogue has to seem like something that character would say, especially in conversation.
Have you ever watched a TV show that makes you wonder where the hell that line came from? It's the same with your book. I'd argue that a reader will be more attuned to it than a program viewer. Why? Because they should know the character inside out when reading. Thus, they will catch those nasty little bits and miss the point of whatever you were going for.
What sounds natural? Go and have a listen to a few conversations from strangers nearby. Please don't be a creep about it, and please don't jump into their convo, just don't. Jot down a thing or two that you might use later and then play with the sentences for practice. Write a question down, then try to think of how you might respond to it or how somebody in different situations would respond to it. Practice makes perfect.
Use everyday concerns to create something natural. For example: Have you and your significant other painstakingly tried to decide on something to eat? In the first chapter of 'A Whisper of Wind', I use this model to form a dynamic relationship between the two characters in focus. People relate to that stuff, so use it where it makes sense.
DO NOT FORCE IT!!! Nothing is worse than a forced line to align with the author's intention for the scene. If it doesn't fit, it doesn't fit, and you may have to throw it out entirely. Yes, it is painful to do that, but for the sake of your story, it can be vital. In real life, people will give you the funniest stares if you force something into a conversation. Everyone says to "be yourself" for a reason; it's no different for your character!