I would suggest using foam and painting it. Since this is your first costume, I wouldn't suggest covering it with fabric.
Fun foam (also called craft foam, funky foam, and L600) can be purchased from craft and fabric stores (like Joann's), and although a buck fifty a sheet doesn't seem like much, you will have to buy a lot, so I would suggest finding a specialty foam store in your area or online you that can purchase from for a much better price.
As for cardboard, it can be useful if you know how to use it. If you're using foam, though, you really don't need it... if you want to strengthen a piece of foam, double it up or glue some styrene to the back.
You might want to check out the
foam subforum.
If you're having trouble figuring out where to start, I'd suggest that you get out some paper and a pencil and start drawing the hell out of Ravage. Draw him from every angle, draw exploded views, draw individual parts that will require more detail, and draw how you want the costume to fit on you.