Does anyone have suggestions for crafting realistic, or at least plausible, dialogue?

I enjoy writing, and I can craft a story well enough, and imagery, (when I try), but dialogue is my weakest point, and also one of the most vital for crafting believable characters.

What I had most in mind were screenplays and film, because dialogue is most necessary there, but I’m also interested in plain-text stories, and even graphic novels, where it’s also very much necessary. I was wondering if there was anything I should keep in mind, or take note of when talking to anybody, or while observing them outside of dialogue, like body language.

Basically, I want some advice so I could experiment further on my own, because I just need something to push myself into actually giving it a better effort. Anything would be appreciated!