First off, welcome to the 405th!
It's all about learning and practicing. About 6 months ago, i knew nothing about fiberglass, pepakura, or even halo costuming at all, but now i have a suit of mark Vi armor, ODST armor, and UNSC marine armor sitting in my room. One thing that helped me a LOT was youtube videos. Before i joined the 405th, that was all i had, but theres TONS of great tutorials as well as some videos of people just showing of their armor in general.
Building a helmet out of EVA foam is not very common, or easy. Most of us use pepakura for the helmet, even if you are making the rest of the suit from foam.
The most inportant thing is to read, read, and read some more! Make sure to read the "stickies" at the top of the page in the n00b forums. Theres lots of information there, including forum rules, a list of general supplies for making armor and much more.
Other than that, just have fun! It doesnt matter what anyone else thinks of your props, as long as you had fun and learned something by making it!