Asardes provides good info. I'll expand with some basics to address your armor rating question.

How much incoming damage you mitigate is a factor of multiple components, including armor rating and the character's physical/spell resistance. However, things are very wonky now because of One Tamriel. In order to give you the power to fight c160 monsters throughout the world, your character gets a number of handicaps. These include buffing armor and resistance. In the old days a level 15 cuirass has like 125 armor...you'd be one-shotted by everything if this was still the case. So now the system looks at you and brings your stats up based on your level. This makes the system a little harder to learn, IMO. Your armor rating doesn't change much between levels 1-50. But there are a few helpful things to know:

- as you level, your handicaps fall off, including the ones for armor. So if you keep wearing a level 20 set of gear as you progress from player level 20 to 25 to 30, the gear will get exponentially weaker. Try to keep your gear within five levels of the player level or you could get really squishy.
- The system creates a new, annoying dip in the power curve. When you get to the 40s you lose a lot of handicaps, and this feels like the weakest point in a character's life. If you get into your 40s and feel less powerful and squishy, don't worry...you'll hit vet level and your power will leap significantly.

Mundus stones

Don't worry too much about these. For the most part, you can choose regen for your entire time at non-vet levels. It's the most helpful. Then someday you might want to switch over, but a change isn't that helpful until you're using end game gear. An exception is that you might be a dragon knight or stam Templar based on the heavy armor. If you always have enough resources you might want a damage mundus like asardes mentions.

Traits

Armor traits aren't too important in early levels, either. Most players go with either divines or infused. Infused gives you a bigger enchantment on the gear, therefore more max resources, therefore you hit harder and can cast more. A third option is training. This increases XP for monster kills...leveling you, your skills lines and your abilities faster. If you ever get to a situation where you want to grind raw xp, like working for champion points faster when you're a vet, a training gear set is very helpful.

Armor weight

You mention using heavy armor, so a tip about that. In ESO, the armor line passives work so that heavy makes you tankier, medium makes your stamina hits more powerful and magic makes your magic hits better. Many players in endgame wear a blend. A lot of stam builds use 5 medium and 2 heavy. A lot of magic use 5 light and 2 heavy.

It's a good idea to level two of the armor lines to fifty, if not all three. Throw on 1-2 pieces when you're at lower levels, and then the skill lines will level and be ready to use if you ever need to mix up your armor.

As as a heavy armor wearer, if you ever find yourself way tanky but with low damage, consider moving to another weight - a 5 medium build if you're stam or 5 light if you're magic. I know a lot of new players who get frustrated playing dragon knight heavy armor builds. Yes, in end game content and pvp they need heavy, so they need it leveled and skill points spent in those passives. But in everyday trash fights they can be BORING, lol, and might benefit from dps over resistance.

Hope that helps. Last thing is I'd recommend the websites ESO Academy and Tamriel Foundry for all sorts of information about leveling and mechanics. People always help in these forums, but sometimes their guides are more comprehensive and faster.