umm...here's a guide I made a while back for someone else. I'll add upon it as I go, too: it's really long, so brace yourself!
not all of this will apply to you, but i'll taylor it for you a little bit.
since our archive is down, you may need to wait a week or so to get the files you need, or if you were a member before our move, you can log onto the old site and get em' there. or I can get them for you, if you can't.
so, here's a list of tips from my own experience:
1: for first builds, it's best to go with EVA foam. foam is usually cleaner, and if you want hardened armor, you can still harden foam with casting resin. I would not suggest hardening for first builds, though.
2: for helmets, either 3D printing, or pepakura is best. when you use foam, since the material is thick, it can give the helmet a bobble-headed, or warped look just to make it fit properly.
3: for foam; use thick (floormat) foam for the base. if the foam is too thin, it can cause structural integrity issues, and warp the part. for detailing, I use both a woodburner and craft foam from Hobby Lobby to get a detailed part. just remember that if you use a woodburner on EVA foam, get a respirator, and use it outside. EVA releases toxic fumes when heated.
I also detail foam by engraving the foam with a razor blade, then use a heat gun on the surface. this will open up the cut, and give some fine details.
4: when using pepakura, make sure when you use Bondo to always use a respirator. it's toxic.
5: when using pepakura and sanding, it's really nice to have a detail sander, this thing can make the sanding process much less painful, and way faster. I use a Black and Decker 'Mouse' and it works amazing, the mouse has a little extending tip to get little tiny details. (also get a dust mask, the mouse has a dust catcher, but it doesn't work very well)
6: when starting out with deciding what armor you want, start kinda simple, and start a test piece of armor. these test pieces can teach you some valuable information and techniques. experiment with your test piece, and try anything you want. you don't want to accidentally mess up a perfectly good armor part when you decide you want to try something.
I did this once, and I had a bad day...
7: when applying
any type of additive (such as Bondo, Epoxy, Etc.)
apply the minimal amount. it's WAY harder to sand material than it is to apply it. I did this VERY recently, and I was sanding for hours. I'll try to save you from the same fate.
8: if you decide to buy an undersuit, get one with a zipper in the FRONT.
if you have to pee during a convention, you will have to take ALL your armor off before you can go pee. you might even pee in your undersuit if you wait too long...
You've been warned!
9: pay good attention to other people's build threads. they will usually share good Ideas about how to go about making certain parts of a build.
it could come in handy. just sayin'
10: if you need anything, just let me know. I'm here to help people get the best out of this site, and that's exactly what I'm gonna do.

also, if you want cool 3D models for 3D printing, tell me! I'm working on a whole arsenal of Halo 5 stuff for the community. if you'd like to suggest one, let me know!
11: ask questions, be curious on threads. If you want to know something, ask the thread owner, and you may learn something new.
12: for cutting foam, keep your knife super sharp, especially if you want to make angled edges. they will get dull fast, so keep em' sharp!
best of luck!