This is more of an opinion or preference question but I was wondering, is it better to show the main characters of my novel first meeting and becoming friends or is it perfectly fine to start off when they have already become friends? What would readers prefer?
Generally it's a bad idea to do anything that does not serve a purpose for the story. Your readers will wonder why you spent their time on it.
If how these characters meet and become friends matters for the story you're trying to tell, show it. If not, don't.
The writers of Avatar: The Last Airbender had to show Aang becoming friends with Korra, Sokka, and eventually Zuko because they help Aang and he helps them grow as characters. The story is heavily about their personalities and how they change.
Tolkien never explains in Lord of the Rings how exactly Merry, Pippin, Sam, and Frodo all met. Unless I'm mistaken, all we get is that Sam works for Frodo. How that arrangement started is left to the imagination. It didn't matter for the story he was telling. How exactly they all became friends had no effect on their trip to throw the Ring into the fires of Mount Doom.
The big difference I think between these stories is that A:TLA is heavily character-driven, while Lord of the Rings is plot-driven. The characters in A:TLA change, the characters in LotR mostly don't.
One question you should ask yourself that may help you decide is: "Is my story about who my characters are, or what they do?" I hope you can see that these are not necessarily the same thing.