First, there are some really broken things which make the game kind of silly (depending on how you use them) unless you crank the difficulty up. I'd personally suggest avoiding using Spell Sequencers/Triggers and Contingencies as well as Traps, or at least restricting how you use them, because you can do some really stupid stuff with them which makes the game a breeze. There are also a few items and enemy vulnerabilities which you can abuse but obviously that falls into the domain of spoilers so I'll leave you to find them out yourself.

Second, there are often non-quest related areas with good loot--make sure you check your map (M) as they'll usually be denoted by pins and have relatively vague names. Also, be sure you press Tab a lot as it reveals lootable areas (also, if you use it while moused over a character, it will reveal that character's name/health instantly rather than having to wait however many seconds).

Third, XdY means "roll X Y-sided dice", so for example if a weapon does "2d4 + 1" it deals 2 4-sided die rolls plus one damage, or 3-9 damage.

Fourth, THAC0 and AC are used to determine if a character hits--when your character attacks, roll a D20 and subtract their THAC0 (To Hit at Armour Class Zero), then add your opponent's AC (Armour Class). If the resulting number is at least 0, you hit.

For example--an enemy with THAC0 12 takes a swing at your character with AC5.

d20 - 12 + 5 must be at least zero, so the enemy must roll an 7 or more to hit (70% chance to hit). Thus lower (and preferably negative) AC and THAC0 values are better.

--

Armour types don't usually work better on certain types of classes because, unless you installed mods which modify how items work (and don't do this on your first playthrough) your characters won't be allowed to use armour types which would hinder them (for example, mages aren't allowed to wear anything other than robes). The one exception is a dual classed X/mage (for example, Imoen or Aerie)--if you equip armour that their other class (Thief and Cleric respectively) allows, it will disable their arcane (mage) spells (so don't equip anything other than robes on them).

--

It took me a run or two through the IE games, largely BG1 for myself, to get used to recognizing how to compare how good various weapons are. Normal difficulty on BG2 assumes you are new, though (it's cranked down from the default BG1 difficulty level because BG1 was ridiculously hard as a new player), so you should be able to get by with just using whatever you are proficient in and has a bigger number; for a new player it's probably safe to just assume a +2 weapon is better than a +1 weapon unless the +1 weapon has some insanely good ability that is quite obviously insanely good to you.

--

Three things which may otherwise take a lot of pain to realize:

1) BG2 is heavily magic-based, much more so than any other D&D game I've played, in that it has a lot of protection magic and countermagic interaction. You'll pretty much need at least one mage in your party with a good number of counterspells, and ideally a cleric or inquisitor with Dispel Magic as well (I use Keldorn if I play Good for Dispel Magic and some spoilerific stuff), since enemies will cast nasty spells like Protection from Magic Weapons, Stoneskin, [Improved] Mantle, Absolute Immunity, etc, which will soak up damage (stoneskin) or block your weapons entirely (the others) unless dispelled, and spells like [Minor] Globe of Invulnerability, [Minor] Spell Turning, Spell Trap, etc which will block your offensive spells. Never leave home without a few copies of Breach (takes down anti-fighter protections) and one or more of Secret Word/Ruby Ray of Reversal/Khelben's Warding Whip/Pierce Magic/Pierce Shield/Spellstrike (take down anti-mage protections, usually redundant but there are a few fights which are pretty much impossible without them and they do tend to be helpful nonetheless) and be sure to use protective magic of your own.

2) It's tempting to see all the Cure {Wounds} spells and think of Clerics as a healer, but in reality they're support casters and melee fighters first and foremost. In fact, I would suggest not bothering with healing spells other than Heal (because it's amazing) and Cure Light Wounds (because level 1 cleric spells are otherwise pretty bad in BG2 and a bunch of Cure Light Wounds will heal a fighter or two at the very least whenever you rest, at least until you get high enough level to get a copy of Heal or two). Be sure to read through Clerics' other spells, especially their buffs (both self and party-wide).

3) It's tempting to just memorize a bunch of fireballs and other offensive spells on your mage (other than the one or two protective/antiprotective spells listed above), but some of the buff/crowd control spells are incredibly strong. Oftentimes a well-placed Web will do more than a fireball ever can.

--

Wow, that was a lot more text than I planned.

According to DanSimpson's guide, you must roll the d20 and the number must be greater than your THAC0 minus the enemy's AC.

So in your example, it is indeed correct, but not the way I'd grown used to seeing it.

1d4 is anywhere from 1-4 damage, it means 1 dice up to four damage, basically, so the max damage you can do is by multiplying the two numbers together, minimum is whatever the first number is. So like a 2D4 is 2-8 damage, etc.

Yes

Not specifically, but mages can't wear anything but robes (or armor specifically noted to be worn by mages) and thieves can't wear anything but leather. Different classes have different restrictions, but other than weight restrictions lower AC is always better (except for any special abilities/conditions you may get from the equip).

General hints:
ALWAYS pay attention to the combat messages you get. Sometimes enemies are immune to certain types of attacks. You won't know what unless you pay attention.

Magic missile and melfs acid arrow are great for distrupting enemy spell casting. Magic missile is near instant, meaning you can cast it once the enemy starts casting, and acid gives the enemy constant 1 HP of damage, which interrupts their casting every round.

Make sure you have a balanced group. Sometime's melee is necessary, sometimes ranged is better, and sometimes magic is the only way to go. And thieves are needed for lockpicking for the most part, but they're also fun and good for stealth scouting.

Save fairly often. No matter how prepared you are, you will come across opponents that your current set up is not able to handle. For example, a cocatrice when you do not have protection from petrification or stone to flesh, you'll probably lose every time. Or if you run up against a powerful undead lich without some good spells like sunlight, protection from evil, etc. The game at times can be very rock paper scissors, you may have to die once, know what you need, and come back better prepared.

Different way of saying the same thing (although I think it's >=THAC0-AC. Might be wrong on that point)

Your formula:

d20 >= THAC0 - AC

Subtract THAC0 from both sides

d20 - THAC0 >= - AC

add AC to both sides

d20 - THAC0 + AC >= 0

I figured that my way of saying it would be easier to understand, but both work.

OR instead of trying to understand these people arguing about the rules, you could read Dan Simpson's excellent AD&D rules FAQ:
http://www.gamefaqs.com/pc/919881-baldurs-gate-ii-the-collection/faqs/8566

I also recommend SixofSpades' spoiler-free guide, which helps you understand the game with NO SPOILERS.
http://www.pocketplane.net/volothamp/bgguide.htm

Thanks all very informative. It has helped me on my play through =)