Well there are many many ways to make them. I have seen guys use MDF board, Plexi glass, and even resin/bondo covered foam board. Check out Vinmans templates, he has some for the gauntlet and the blades. Since you are 14 you are probably on a tight budget, so I would say try and hit a dollar store and look for plastic cups that will fit over your hands( these will be used to make the actual arm part) as well as try to find some clip boards (plactic or wood makes no difference) and some little plastic rectangular boxes. Trace the template for the blades onto the clipboard and use a dremel tool to cut them out. If you don't have a dremmel you can always use bondo (available at most walmarts in the automotive section for about 15 bucks) (WARNING USE OUTSIDE AND WEAR PROTECTIVE MASK!!!!! the fumes are toxic) trace the blades onto cardboard or foamcore and then coat with the bondo.
Once the blades are coated sand them smooth, and paint in a metallic color. Add blood it you like.....
attach the blades to one of the boxes.
Now the gauntlet, cut off the bottom of the cup and glue the box to it. trace the gauntlet template onto some cardstock, foamies, or cardboard and attach them to the cup.
Easy Gaunts.