I had moderate sewing skills when I first started. My grandma taught me to use a sewing machine, and helped me make a couple of very simple things for halloween when I was little and she used to babysit my sister and me. My grandma was very into crafts, sewing, quilting, knitting, etc. so it was something to pass the time with children I guess.
I also had a friend in university who knew a thing or two about making her own halloween costumes, but neither of us had much experience or skill. We could just do the bare minimum to pull together altered thrift store finds to make other things.
As I started getting more into cosplay and costuming, I decided to take some fashion design classes at a local community college that offered a continuing education program with classes on evenings and weekends so that I could schedule around work. It was definitely worth it. I now know a lot more about pattern drafting, apparel construction techniques, and design illustration.
If you don't have the option of taking classes, I'd suggest picking up this sewing technique book: Complete Guide to Sewing
It has photos and step-by-step instructions on various sewing techniques.
As for the armor question, as mentioned before, there's a whole forum section here on cosplay armor and props. And you can also pop on over to TheRPF.com
for even more detailed answers, tips, and advice on armor construction. Look up STEALTH on youtube for video tutorials (he used Iron Man as an example, just so you know which series of vids to watch). The RPF also has a library of sorts of patterns for video game armor you mentioned. A lot of people have already done Halo and Mass Effect suits that you could get way more information about on those forums than you'd expect here.