If you want to build an armor, you have to think of several aspects before you get your hands dirty, you want something "wearable" right?

The first thing you should have is your idea set up, either written on a piece of paper, or actual images, i've seen all, and each person has they're way to work with, but you should have a "basic" concept done, evan if you later choose to build as you go.

After having the concept, you should get a base character model wich you want your armor to fit with, this will not only help create the armor, but it will make sure the armor actually fits on something-

With these 2 things, you can finally start building your armor. Make a copy of the base character model and delete the unwanted faces (for example: delete all the faces besides legs ones), and from there just basically extrude, cut, move vertexs, etc. in order to achieve what you want the armor to look like, do the same for the other pieces of the armor.

That's how most people (i've seen) do to build armors and pieces of cloths, and the way i use it myself.

There are some tutorials in here that might help you preatty well, just give them a go too