In the Armor category you can find the following armor types that are present in the Elder Scrolls Online: Light Armor, Medium Armor and Heavy Armor.
To unlock these skill-lines on your character you have to find and wear some pieces of the armor type, afterwards the skill-line will be fully visible in your character skills section. In case you are wearing only one piece of a certain type of armor it won't unlock the skill-line in your characters skill section, you have to wear several pieces to do that.
The armor skill-lines are leveled up by simply getting experience points from any activity such as killing monsters or completing quests. To level up the skills from the armor type of your choice, you have to place them on your front bar.