In Mission 1 "Dawn", once you gain control of Master Chief, walk out into the next room and instead of going straight on, look to the left of the walkway to see a flight of stairs. Go down these stairs and turn right to the control panel underneath a spotlight containing the Chief's records. Simply hold
on the panel and the 20g is all yours.
Here's a quick video on where exactly the records are held.
For more Halo 4
video guides, check out this link here - http://tinyurl.com/halo4guides