When to start writing a comic

Well, I have this basic idea for a superhero comic. I have most of it figured out, I just need to flesh out things like character backstories and the overall theme of the comic. Could I theoretically start writing this comic and figure out those things later, I do I need to have a certain amount of things planned beforehand?

Like, do I need to come up with the character's personal history that defines who they are before they can appear?

It very much depends on how you like to do things and what kind of comic you intend to write. For instance, if you want to do some kind of heavily Myth Arc-based thing, then it probably is best to establish some of the backstory right away, if not necessarily all of it. Works that are planned out in advance also tend to have a lot fewer problems with Fridge Logic when it comes to doing particular kinds of stories.

However, even in a case like this, you can certainly develop the character on the fly - the best characters will develop as you write them, so trying to plan out everything about them before even introducing them to the comic is not a particularly good idea, no.