I keep seeing a lot of comments from new or returning players that make it sound like they're struggling with character development so I decided to write this post with some basic info that should help. I should also say right off the bat that there are many viable builds that deviate heavily from what I'm writing below but you should leave those until later when you're more familiar with the game. Learn how to walk before you start running.
1. Stamina, Magicka or Health and how much of it.
ESO has a hybrid class/open skill system with heavy emphasis on the open. Picking a race and class is not even the most important choice you'll be making. All characters are more defined by which one of the three basic resource attributes you emphasize. It's that choice that will determine what type of armor you'll wear and what weapons you'll be using.
Stamina is the choice you make if you want to use 2HD weapons or dual wield or use a bow. The damage that the abilities in those skill lines do is determined by your total Stamina pool and Weapon power. Those are the key stats you'll want on your gear to do more damage.
Magicka and spell power do the same for the staves: both the destruction and healing ones. Those are you key stats for both magcka based DPS as well as healing,
Health is the resource of mitigation and tanks. Just be aware that if you chose to emphasize that, it'll be at the expense of your damage output. It's actually tough to try to build a tank in the early levels while you're mostly solo questing. Also, tanks in the normal version of dungeons are really not 100% needed. Tough veteran dungeons and trials yes - tanks are very much needed there but that comes much, much later. If you're still determined on building for tanking right from level 1 just be aware that your time to kill stuff will be very slow compared to others.
If you read guides with an emphasis on end-game you'll see recommendations to put all your points into either stamina or magicka and nothing in health (unless you're building a tank where you'll want to have roughly an even split between health and you chosen damage stat - I won't even get into the details that tanks can also be magicka or stamina variants since that's a more advanced discussion.) However I think that's the wrong advice for character leveling especially for new players. I recommend that you do a 4:1 magicka or stamina to health ratio to give you a little bit of extra health. But in any case putting points into all 3 is almost always a very bad choice.
2. What armor to wear and about that jewelry.
Light armor is for magicka builds, medium is for stamina and heavy is for tanking - it's that simple. Aside from champion points which come after level 50, it's the passives in the armor skill lines that increase your chosen stat pool, increase its regeneration and give you bonuses to the type of damage or healing you're trying to emphasize.
Armor also determines your physical damage mitigation to a large extent from hardly none to quite a bit, but light armor actually gives you slightly better spell damage mitigation than heavy.
Most of the bonuses you get from the armor passives is on a per piece basis. Most of the time you'll want to have 7/7 pieces of the same type to maximize those bonuses. There are special cases and special situations where you'll want to go lower than 7/7 (PVP, some tank builds, etc.) but as a newish player trying to figure things out you should always be aiming for 7/7.
Jewelry also typically comes in 3 flavors: one for magicka, stamina or health.
Something else that has always been in the game but has been emphasized tremendously with the latest One Tamriel update is weapon, armor and jewelry sets that give you special bonuses depending on how many pieces of the set you've equipped. They drop everywhere now and you get them as quest rewards. They are specific to each zone or each 4-man dungeon with each zone and 4-ma dungeon having 3 sets: a light one for magicka, medium for stamina and heavy for tanking. It'll be tough for you to play the game and not get full 5 piece sets useful to you.
And also note that with the new system where you're always bolstered to CP160 if you're under that, the level of your gear relative to your true level has an impact on the bolstering formula - you generally want to be using gear that isn't more than 5 levels below yours. Don't sweat it too much as you'll get lots of replacements. Just be aware of that.
3. The right weapon
If you're interested in stamina-based DPS, 2HD, dual wielding and bows are your three choices. For magicka DPS it's just the destruction staves (although I'll mention here that there are some advantages to dual wielding swords for magicka DPS due to a DW passive but that's end-game min/maxing - don't sweat that either while leveling) and for healing it's the restoration staff. 1-HD + shield is purely a tanking choice and there's very little in that skill line that will increase your damage.
A note about weapon and spell power: passives and gear stats that increase your weapon power will only ever increase the damage of the stamina-based weapons and their abilities - never either type of staff: you want spell power to increase that.
The reason you'll want to stick to those choices instead of mixing and matching (even though you can) is very simple: they derive their damage from opposite pools (magicka or stamina) and the bigger that pool the better the damage. If you pick one of each type you'll either be doing poor damage with one, or worse, with both if you decide to split your stat allocations. Experiment if you want but at least you'll know why your bow abilities suck when used on a magicka build and the same for a staff used with a stamina build.
4. Food buffs
If you're only going to craft one thing, provisioning should be your choice. Food can buff one of your 3 stat pools if bought from NPCs or is green quality crafted, 2 of those pools if blue crafted or all 3 if purple crafted. If you're going to be crafting your own or buying it from other players, the blue kind that buffs your health + your choice of magicka or stamina is the one most people use. The purple does buff all 3 but for a less amount than the blue food does.
Drinks work the same way but they don't buff the pools, they just buff the regeneration rate. It's why most people don't use them except in special builds that already have a ton of the relevant stat pool without adding food.
Always use food: the buffs are big.
5. Ability morphs and ultimates
The skill lines for all 4 classes start as magicka abilities and so were the morphs for the first few months of ESO. It hasn't been that way for quite a while now and all classes have several skills that morph the skill into a stamina based one using all the same stat criteria as the weapon skills I described above (stamina + weapon power for the stamina morphs.) All 4 classes have viable stamina or magicka based variants now.
The ultimate abilities were also changed around the same time to get their power from whichever of your 2 stats is larger, weapon or spell power. They can be used just as effectively by magicka or stamina variants.
6. Choice of Race
Although hardcore min/maxers will disagree, pick whatever you want. Having said that, these are typically acknowledged as the best choices for different build types:
Magicka DPS: High Elf, Breton, Dark Elf (especially this 3rd one for DK fire based magicka builds) Stamina DPS: Redgard, Orc, Khajiit, Imperial Tanks: Imperial, Nord, Orc Healers: Breton, High Elves
“That's how the madness of the world tries to colonize you: from the outside in, forcing you to live in its reality.” ― Jeff VanderMeer, Annihilation