This is a very basic guide for people who are relatively new to the game or new to end game.
Skills & Stats:
Choosing what skills/stats to use when dps'ing or even healing can be relative to what role you play. As a healer you'll generally stick to Magicka abilities/stats, as a dps it can vary, and as a tank you'll probably use a range of both.
Example: If your stat points are Magicka, you wont get decent damage if you're slotting Stamina spells. (FYI Having half magicka and half stamina generally means you'll do 50% less damage regardless)
It's a good idea to have all of your stats in one type (magicka/stamina) unless you're a tank, then you can mix it up a little.
Skills & Gear:
Choosing what set pieces to wear can be a tough choice, but at the basic core of things, if you're a dps(or a healer) who uses Magicka abilities, you want to be wearing at least 5 pieces of light armor to get the correct bonuses to help your damage/regen. If you're using Stamina abilities you want to wear 5 pieces of medium armor, and if you're a tank you generally want to wear 5 pieces of heavy armor. You can wear more pieces if you haven't got the undaunted passives (explained in other guides) but generally speaking you should never have less than 5 of your desired role (Tank/Mag dps/Stam dps/Healer)
Skills & Food Buffs:
If you're a tank, I suggest using purple tri stat food (magicka/stamina/health). If you're a magicka dps I would suggest using max magicka/health food. If you're a stamina dps I would suggest using max stamina/health food. You can always mix it up a little, but you're always going to need to be above 15-16k health to not get absolutely destroyed by 90% of the mechanics in veteran dungeons. I would not suggest using regen food that doesn't give you health and your primary stat.
Skills & Glyphs/Enchants:
Generally speaking most gear you equip (asides from crafted gear) will come with enchants already relevant to the role you're playing (Magicka on cloth, Stamina on medium etc), however sometimes it might be that you're extremely low on health even with a max health/magicka food slotted(for example). It may be that you want to slot a health enchant onto your helm or even your chest to make sure you can survive some of the bosses mechanics. As a rule of thumb, you don't want to be any higher than 20k health(unless you're a tank) buffed as a maximum(as it will lower your damage/healing), I would suggest if you are that high in health, slotting instead the glyphs/enchants onto your gear relevant to your dps type.
Front Bar & Back Bar:
Unlike other mmos, ESO moves have no cooldowns, and there are only 5 abilities you can put on your bar. Which means that to fully utilize the options available for doing damage/healing and tanking, you need to properly set up your backbar and rotate while fighting to maximize healing/mitigation/damage. Below are some common suggestions for common roles.
Tank - Front bar=One hand and Shield. Back bar= One hand and Shield. Simply put, the reason for this is simple, if the boss is about to smash your brains in, and you're bar swapping, you don't want to get hammered with a resto staff out.
Magicka dps - Front bar=Destro Staff. Back bar= Destro Staff. The reason for this is sadly, there is no good alternative, the only damage option magicka is presented with is Destruction Staff. You won't do any decent damage with Stamina weapons and your healing isn't what you're there for if you're a dps.
Healer - Front bar=Resto Staff. Back bar= Destro Staff. Healing generally doesn't take up too many skills, healing springs, breath of life, combat prayer. The reason for the destro staff backbar is to debuff/boost the dps of your group by lowering the resistances/damage of the boss. Ele drain is a key skill.
Stamina dps - Front bar=Two Handed or Dual Wield. Backbar= Bow. Generally the best "DoT" damage you'll get for easy backbar rotation will be using a bow.
Simple basic rules for being in a normal or veteran dungeon - Keep your food on, have above 15-16k health minimum. Have pots (Even basic pots). Be polite (the simplest thing you can do in a dungeon is have good manners, a lot of people appreciate that more than you might realize). Be active (don't afk all the time)
Final Note - If you're there as a tank, remember your job is to keep yourself alive, and keep the boss/hard mobs taunted. Your role isn't to worry about healing but to mitigate your own damage. If you're there as a dps, you want to do as much single target or aoe damage as possible to make the fight quick for the healer. If you're there as a healer your role is to keep the group up and debuff the mobs/buff your group.