dwoo's reply has some good pointers to get you started - but there are some other questions you should ask yourself before you get too carried away:
1. How big is your budget? Suits can range in costs from $30 (TastyBread's cardboard suit) to $400+ for a wacked-out pep suit. How much money you're able to spend will dictate a lot of your design decisions.
2. You're 13. Will your folks let you play with fiberglass? That's a big issue for the younger builders here - so you may be using cardboard reinforcement, or styrene sheets, or foam board, or what have you.
3. Do you have room to spread out? For several weeks? I have stuff strewn all over my house - office, living room, back deck, kitchen (I live alone, so I can do that). For instance, my resined armor is hogging one corner in my living room, I've got tools and equipment occupying my kitchen table, and various odds & ends floating around in the other rooms. Do you have pets? Annoying little brothers who absolutely HAVE to touch, touch touch (even curing resin, which sticks to everything)? The amount of space you have to work with can affect both your build efficiency and the "Project Irritation : Owning Own Armor Glory" ratio in a negative direction.
4. Plan, plan plan! What are you going to use for straps to keep your armor on? How are you going to attach the boot pieces to your shoes? What are you planning to use for the visor? Do you want the polygon look or a smooth molded look (one requires a lot of sanding, one doesn't). Each build is specific to the person, and you have to be thinking about these things in advance so you're not tacking things on at the last minute and ending up with a result you aren't happy with.
5. Do you have patience and attention to detail? A MJOLNIR suit is a big undertaking, even a cardboard one. You have to be committed, so be upfront with yourself on this one. You might be better off with something a little less complicated...like a mummy costume or something.
And lastly, there are no step-by-step instructions. There are great pinned tutorials for various aspects of the build, countless tips and tricks in the many topics, and a lot of helpful people who will gladly point the way if you get stuck or have intelligent questions to ask. MJOLNIR armor is like a huge puzzle - you have to figure out some things for yourself. Which makes the end result that much more satisfying, imo. But nobody is going to hold your hand and do it for you...unless you get your dad to do it.