how do you unlock the helmet in the armory that says you need to unlock it through halo waypoint?
You will need to download (if you haven’t already) Halo Waypoint, then there will be certain challenges through Waypoint (various Halo achievements) that you will need to unlock, then visit Waypoint after completing those tasks, and the armor will be unlocked.
Hope this helps. 
Waypoint and playing Halo 3 by completing certain challenges.
There are achievements from Halo 3 and odst that when unlocked unlock armour in reach. Also some of these armoury items require a certain halo score, such as the security armour, it requires 30 halo score and the first strike medal from a multiplayer matchmaking game. Some achievements include finishing halo 3 on legendary and odst on legendary as well, or getting a certain number of needler kills in either games.