Hello there! Welcome to the 405th!
For me, I chose a starting point and goal. The most notable armor for me, is my Reach Operator set. My clan members knew me by that set, and I still rock it in 2019 on H5. A few years back I started with Pepakura and had quite a few pieces done for my set. It was a great learning experience and I decided to scrap it in the end. Now, I'm starting back up with a foam build, still using pepakura for the templates, but using different materials for the build.
My suggestion is check out the forums for tutorials on using Pepakura and for foam builds. There are quite a few, although some techniques have probably improved as the cosplay community is constantly evolving. There are also many wonderful build posts you can check out to see what materials other builders are using. You can also check out Youtube, some especially helpful people I've found are AndrewDFT, PunishedProps, EvilTed, and KamuiCosplay (she has some full process videos of a few foam based cosplays and some product testing videos, very insightful).
Then, when you've decided on an armor, you can check out the Armory here for specific files for Pepakura. Using Pepakura Designer/Viewer takes a little poking around, but you'll catch on. You can download it for free, but you'll need to purchase if you want to save your scaling/editing.
A good starting point on actually building would be, if you're going the foam route, find a small simple piece (like the reach handplate for instance), scale and print from the Pepakura file, transfer to foam, cut out and sand down/seal/paint with any techniques you're interested in using.
Good luck, happy building!