I'd personally go read something about the storyline...better yet, i'll give you something to read.
That will surely fill you in on anything (Including the novels) you need to know before Halo 3. There's also some videos with all the cutscenes from Halo 1 and 2 in there. So dig in!
And i'd join you all on your offer, but at the moment the 360 isn't connected to the internet/Xbox Live or anything. I need a wireless router and adapter as my computer isn't anywhere near my 360...