@Loud Angry Dead: You'd be welcome to submit a character at any time. As Shien pointed out, reading the entire Compendium isn't necessary; it's usually better used for reference, if knowledge of one point or another becomes relevant at a given time. Or you could ask me (or Shien - she's usually faster at replying than me, and is in possession of an almost encyclopedic memory), if you have questions.
Hmm... I think at least the time-section of the Time and Ages-article would be considered prerequisite knowledge, which is fine since it's not very long. Taking a gander at the Map of relevant areas of Kirirak would probably also be a good idea, found in the Regions of the World-article. Oh, and the passage about races, obviously. (Eh, there's supposed to be a map of Rodoria in there, too, but it doesn't seem to have been added yet. Shien?)
Aside from that it would depend on what your character knows and what is relevant knowledge in terms of its skillset. Making a mage or Favored One (ones that invoke powers bestowed by deities) would probably require reading the article on magic; in case of a Favored One, the particular powers granted (and conditions for receiving those powers) would also depend on the chosen deity, so the article on deities would be needed. Just skimming magic and deities would actually be a good idea anyways, now that I think about it.
Those are the parts most vital to making a character, I think.

@Shienvien: Right, I'll start thinking about that a bit more earnestly, then.