Frankly reading people's posts and asking questions is a perfect place to start. Also be sure to check out the
Tutorial Index if you haven't yet.
First off I'll point you in the direction of the
Foam Guide or, if videos are more your thing, check out this
YouTube channel by a certain red Spartan around here.
For foam
DFT has some good beginner-friendly suit builds such as a
Marine and
ODST and various prop builds (
SMG,
M6C SOCOM, and
H5 DMR come to mind) for getting started if you want to follow along step-by-step.
The aforementioned Spartan's channel is doing a Reach suit build tutorial series that'll get you an amazing build and get you used to using pepakura templates for armor pieces, so I'd recommend checking his channel out.
As for what armor to do, only you can decide that. What armor interests you? What part to start with? If you want to practice getting used to foam and the techniques something small like a handplate is the way to go as you'll waste a lot less foam if you need to restart or do something again. If you'll be motivated by having a really cool piece done or think you might not want a full suit, just a show piece, a helmet can be a good option, although foam helmets aren't as common as other methods.
For scaling and printing Armorsmith at $30 USD is a really good option because you can create an avatar with your measurements to get a more accurate estimate as to the scale you need. Pepakura Designer is also a great option that you can start using for free (some features are not available in the free version such as saving) and is $38 USD to purchase a license.
Good luck crafting, and don't hesitate to ask questions. People around here will help in whatever way they can.