Attributes play a large role in determining your effectiveness in combat. All gear has attributes grouped together in what we call attribute combinations or prefixes/suffixes.
LEFT COLUMN
Power - Increases direct damage
Toughness - Increases armor
Vitality - Increases health
Precision - Increases critical hit chance
Ferocity - Increases the amount of damage your critical hits do
Condition Damage - increases condition damage
Expertise - increases length of time conditions last
Concentration - increases length of time boons last
Agony Resistance - refers to a Fractal mechanic
RIGHT COLUMN
Profession attribute - Please review wiki section for this
Armor - Decreases incoming direct damage (Toughness + Defense from armor)
Health - lose this and you die (1 vitality = 10 health)
Critical Hit Chance - Your current chance to score a critical hit (21 precision = 1%)
Critical Damage - how much stronger your critical hits are (15 ferocity = 1%)
Healing Power - Increases all healing your character does
Condition duration - How much longer your conditions will last (15 expertise = 1%)
Boon duration - Increases duration of all boons (15 concentration = 1%)
Magic Find - improves loot quality. Check the wiki article to learn more
Boons are effects that provide benefits. They are stackable, but some stack in intensity (might), while others stack in duration (swiftness). Follow the link to see a chart of boons. Boons can become corrupted by certain skills, which will convert them into a condition. Here's a list of corruptions.
Conditions are exactly what you would think - things that you usually don’t want on you such as bleed, blind, vulnerability. Conditions can be converted into boons by certain skills. Here's a list of conversions.
A concern that many new players have is that they’re afraid of messing up their build. Guild Wars 2 is very forgiving because it’s relatively easy to obtain new gear with the stats you’re looking for and you’re able to swap out your skills/specializations/traits whenever you want to without any cost. You will have enough Hero Points to purchase all skins and specializations. Using a site like MetaBattle and then tweaking it is a good idea.
Skills - You probably noticed that your first 5 skills depend on what weapon(s) you use. For every profession other than Revenant, you get to select a healing skill, 3 utility skills (level 11, 15, and 19), and an elite skill (level 31). If you’re playing anything other than an elementalist or engineer, you should get used to the weapon swap mechanic during combat, as it’ll increase the versatility, utility and damage you’ll be able to provide. As an elementalist, you’ll need to learn how to effectively swap attunements, engineers will be swapping kits, and if you’re a revenant, you’ll have the ability to swap Legends.
Specializations - These are very well-explained on the wiki so you are best off referring to that page. They offer passives that adjust the way certain skills work and ultimately customize your playstyle. Elite specializations take this a step further.
Power or condition damage? You’re looking for a power build if you’re centering around dealing direct damage. If your skills apply a lot of conditions (bleed/burn/poison/confusion/torment) aka stuff that does damage over time, then you’re looking for a condition build. There are other builds, such as support builds, but you likely won’t be using them while leveling unless you always play with a group.
What should I use while leveling? Excellent question. Honestly, it doesn’t matter too much as the leveling content is relatively easy. Stacking as much damage as possible is the general rule in this game, since the best form of damage mitigation is avoiding damage rather than trying to out-tank it. Learn to dodge, reposition, use condition cleanses and stunbreaks, and how to use conditions/crowd control skills effectively and you’ll be able to clear content without wearing any armor if you want to. I would advise checking karma merchants after completing hearts to see what they have. Accessories are often worth grabbing from heart vendors while leveling since the game doesn’t give you those often as rewards for completing story missions/leveling up.
What else should I know about my build? If you’re in the open world, you probably want to make sure you have some form of condition removal, swiftness (especially if you don’t have a mount), a stunbreak and/or stability. In general, familiarize yourself with what boons and combos are and what they do. If you play solo, you’ll want to make sure your build contains skills that provide crowd control (daze, stun, knockdown, pull, knockback, launch, float, sink, fear, taunt, petrified, freeze, blind, chill, cripple, immobile, slow, and weakness). It’ll help keep you alive and some enemies have a defiance bar that must be broken in order to stun them and/or do additional damage to them. Make sure you learn how to use your evades/dodges/blocks/aegis effectively if you have skills that grant those effects.
People often have different builds when completing specific content, such as dungeons, fractals and raids, so research accordingly!
I recommend taking a look at Metabattle's leveling guides section (near the bottom of the list).