Welcome to Gaia! ::

It depends on what material are you interested in doing. Cheapest route is craft foam and good material for first timers. I like using wonderflex and craft foam together.
Before starting, I usually figure out how to put the armour on me; looking for ideas by checking out real armours. If I find one that is close to what I need, I use the idea and build around it.
This is one of the craft foam tutorial when it comes to armour.
http://entropyhouse.com/penwiper/costumes/helmsdeep.html

What material are you thinking of using?

I make chainmaile armor. I doubt that would be something you would want to do for this costume though since it is incredibly time consuming. Took me three years to finish my haulberk (of course that is with that meddlesome "life" thing getting in the way). I think for cosplay foam might be the quickest, cheepest, and lightest option. I can't give much advice for working with foam but I would recommend trying it on often to make sure the sizing is correct.