Lol most of you are idiots. Halo: Reach will take place no more than a month or two BEFORE the beginning of Halo 1. It will more that likely talk about the story of all the other spartans EXCEPT the Master Chief (John 117), Linda or James. It will talk about Red team, the team that dropped to planet reach which consisted of 28 or so spartans. They were sent there to defend the fusion generator for the 20 orbital defense platforms, the Super MAC Guns. That is most likely where this will take place.
As for the Master Chief, Johnson and Jenkins? They may have the mission in orbit where Blue team (John, Linda and James) went to destroy a NAV database on a ship called the 'Circumference'. Blue team finds Johnson and Jenkins fighting a bunch of jackals. But that means the Master Chief will only be in one mission AT MOST.
Go read the book series, its all there.
[Edited on 06.03.2009 5:26 PM PDT]