Are you going for Halo 1 or Halo 2 UNSC Marine? Looks good so far! I tried to do some pieces out of cardboard to get the general idea. I stopped because I don't have the skills to go much further than that. Looking at your chest piece you have the shape and angle down. As you progress you might want to add some thick foam for more volume since the chest portion seem to protrude out a bit.
Next question is are you going to come up with a vest or vest pattern to go underneath the armor? Didn't look like the HALO 2 Marines used this, but if your doing HALO 1 then you will have to think about this. I'll have to find my old post, but I had some sketches of this I posted.
Good luck and keep us informed.