There are some fundamental differences between actual, spoken conversation and the dialog we expect to see in fiction. This means that dialog is one of the most difficult things to make "realistic" in writing, because the idea that we need to make is "realistic" at all is a misapprehension. I'm going to run down a few highlights.
1. People say "um" and "uh." Characters do not.
It's not that characters are that much more intelligent than the average person, but rather that the reader hasn't got time to filter out a lot of "Er... Um. Hm," every time your protagonist needs to order a sandwich. These interjections are one of a set of several things that we add to dialog only as seasoning: a dab here, a pinch there, and you're done. You can throw in one of these to demonstrate that a character is taken aback by something or confused by some strange circumstance. Use too many, however, and what you're signaling is not that something weird is going on, but rather that your character is stupid, apt to be confused by the most mundane things.
I don't bring this up because it's a thing people commonly get wrong--writers rightly see the "hmmmm" in their stream of consciousness as their own grasping for what comes next rather than the character's--but because this concept of "spice" or "seasoning" is a good one to carry forward.
2. People work blue. Characters are PG-13.
If I had to guess how often I used the word "fuck" throughout my college years, I would be too ashamed to actually give you a number. It's dropped off a little since then, believe it or not. This is a thing that real people do: put them in a blue collar situation and they will swear a blue streak up and down the road. Characters don't have this luxury.
"But Tom Clancy's characters swear nonstop!"
You're right; his works are a prominent exception when I think about this topic. But how would his characters express an extreme sentiment in a trying time? Like, what if someone were to drop a bomb on their garage? Well, they'd have pretty much the same reaction to that as they would to sour milk on their Cap'n Crunch. "God dammit to hell, stupid expiration date..."
By all means, let your characters swear, but use some restraint so that profanity doesn't lose its impact. Do that, and it's also possible to portray a character as being "foul-mouthed," a thing that becomes far more difficult to pull off if everyone in your cast is screeching four letter words nonstop.
3. People make small talk. Characters make points.
I've had two people ask me how I'm doing today and a third talk to me about their dog. None of that has a damn thing to do with anything I'm trying to accomplish. It's called small talk, and it's something humans engage in when stuck together on an elevator, or when waiting for a taxi after someone has dropped a bomb on their garage. I assume. Anyway, the point is that your characters should not do this. They should not have time to do this. That's not to say that every line of dialog has to relate directly to your plot. Not every spoken word relates to the Death Star: some of them are about wampas, or nonsensical euphemisms for sex, drugs, and rock and roll, like "Toshi station" and "power converters."
Dialog is one of your most powerful tools. It's highlighted with a pair of quotation marks, it is often offset with whitespace, and it tends to take up less room on the page than your exposition, which means that readers almost always read dialog. (Fun fact: they will skim pretty much everything else!) This makes dialog prime real estate for anything important that you want to say to the reader. Keep your blathering to a minimum. If a little bit of color commentary is important to a character or to the setting, sure, go for it! ...But don't waste my time with a grocery list unless those groceries are about to get MacGuyvered into a miraculous escape or a bomb for someone's garage.
None of this is realistic in the least.
Spend a little time watching Bob Ross on YouTube. He has not once painted a tree, or a rock, or a bush, or a lake, or a mountain, or a cloud, or a sky, or anything at all. He just bangs the brush on the canvas with the right technique, and your brain will tell you that you are looking at all of those things because you've seen them before, and the illusion works. Your objective in writing is very similar: you want to give your reader the illusion of realism (we call it "verisimilitude" when we have extra $64 dollar words lying around). but your writing needs to be better than real.