Read... Read... Then read some more. The stickies on each forum section and the actively updating (and older) build threads to see how others did it. Search for awhile before you ask a question, some likely has asked it before. When that fails, look for someone who is building something similar, and see if they overcame it. When all else fails, make a thread (don't do this first).
With armor, there are two major options: Pepakura, and foam. Pepakura tends to have TON of detail, but has a very lengthy (tedious more than hard) process. Foam tends to be easier to assemble, but a lot of detail is lost and needs to (if you want) be replicated with cutting, paint, or thinner foam. Check their respective forums under the "Costumes & Props" tab at the top of the page.
Another good place to start is with watching a few videos on building armor:
- Evil Ted Smith
Youtube
- SpacemeatTheCosplay
Youtube
- Bill Doran
Youtube (He also has a neat book)
- And so many more.
Also remember that the RPF (link), or Replica Prop Forum is the sister (Mother? Father?) site to the 405th and 501th (Star Wars) forums, you should try checking there as well.
Hope all goes well - Nerdman