The majority of the craftable armor in Knights and Dragons have a plus (+) version which is essentially a better, improved set. The plus version has better starting stats for attack and defense. It also gains more points per level. To create these + versions, you just have to repeatedly level up the normal version until you unlock the + variant. For Level 30 max. armor, that stage is Level 10 (at this point you will have access to the + version). For Level 50 max. armor, the point at which you get it is level 15. For Level 70 armor, the point is level 20.
Once you unlock the plus version, it doesn't mean the armor that you have leveled up automatically becomes +. Rather the next time you craft that armor from scratch, it will be a + version.
The Armorsmith is the place to go to craft armor. This is where you physically create all the armor that your knights will end up wearing. You can also enhance armor, once it has been crafted, which you can read more about in the Armor Enhancement section. Early on in the game, just having one Armorsmith will be fine for your progress. As you get further into the game, however, and your Gold production increases, you get to the stage where you are able to have multiple Armorsmiths creating many sets of armor at a time. These can then be used in the Armor Enhancement process. Once you get more than just a couple of knights you will need to have new armor coming into your stores regularly.