This was one of the first sites I came across when I was learning how to make props. Read it. Read the entire thing. While I don't suggest vacuum forming as a way to make your armor, their overall process is very educational.
The best place to go for information on Power Rangers props and costumes.
This guy is amazing. He makes an incredible looking Guyver suit and helmet. His tutorial section can be applied to almost anything.
This is a great forum where prop makers can show off their work and ask questions. Search through the different sections and you can learn a lot without even having to ask. Lots of knowledge there.
This is where I get my molding and casting supplies. Reasonable prices, and quick shipping.
More links to come.