Is it necessary to the story for these events to be explained in detail? Will they advance or add to your book in any way? If not, I'd recommend against writing a prequel unless it's something that you feel you'd really want to do and would be fun for you. Every book has events that happen before the story begins, and most of the time, they don't need to be elaborated on in a prequel if they can be explained in the first book. Try to fit as many past occurrences that are important for the story and reader's understanding into your current novel (how you reveal it is up to you). If you really think that you can't fit all of it in and really do need a prequel to enhance the book, then write one.
Also, if there are any details in your characters' pasts that don't matter to the story, then cut it. For example, nobody's going to want to know about, and they're especially not going to want a prequel including, that one year your protagonist's best friend worked at an awful retail store if it's not going to tell us more about that character or add to the scene/overall plot. Your readers don't have to know anything about random happenings in your characters' backstories if it won't matter in the story, so if you know that any events you're thinking of adding are redundant or worthless, I recommend against adding them.
Sorry if that was a bit long, but I hope I was able to help! Good luck with your novel.
