Hey Wrath15! Welcome to the 405th!
I can see that quite a few people have given you some advice already on how to get started, but I would like to give ya some more.
First of all, if you are looking for Halo Reach Armor files, the BEST place to really go is here:
http://www.405th.com/showthread.php/24086-*The-Official*-Halo-Reach-Pepakura-Files-Thread
It contains the bulk of the models that have been made for the Reach armor.
Also, there are quite a methods out there for scaling. Some can be confusing if you suck at math or just are not all that good with numbers, so here is a link to a very detailed tutorial which I believe is great for new people:
http://www.405th.com/showthread.php/31627-HaloGoddess-quot-How-To-Scale-quot-Tutorial
It's a tutorial that I made and is the same method that I use. Seems to work every time I measure and my pieces come out just fine.
If you are ever looking for any other files, a lot of them can be found here:
http://halocostuming.wikia.com/wiki/Pepakura_File_Index
Also as a note, you DO NOT need pepakura viewer. You NEED pepakura designer because without it, you will not be able to scale your pieces so that they fit you. I would not even bother with downloading viewer. I've NEVER used it. Viewer would be sort of pointless since you can only change your measurements in designer anyway.
Hope that further helps you out so you can get started on your build.
If you have any other questions or need help, don't hesitate to ask. I've been here for about 2 years now and have 2 builds that I am working on, although my hayabusa is on the backburner for now as I am working on a foam Kat build.