Okay, here’s my (subjective, and other players will disagree) thoughts on classes and spells:
BG3 (like DOS2) wants you to have a balanced party. The things you need are:
- High dex and sleight of hand for lockpicking (Rogues are best, but anyone with the skill and buffs can do it)
- High charisma and persuasion for dialogue
- A melee class with a lot of hit points and high number of attacks
- A magic class with area-of-effect and utility spells
Rogues are awesome, you get the dex you need, stealth, skills etc. (In D&D Bards are even better as they get all the same but with full spells and a ton of skill specialisations, but I don’t think they’re the same in BG3). Rogues are however low armour and hp, and if they can’t sneak attack then their damage goes lower (this is the price for all those useful skills, you’re not as good at melee as a fighter). Arcane Trickster is probably the most powerful one in D&D too.
Barbarian = takes half damage from raging and gets automatic advantage from reckless attack. This often works out just as good as the Fighter’s extra attacks from Action Surge (fighters don’t have advantage so miss a bit more often, but have more attacks to keep trying). The half damage from rage and huge hitpoints makes Barbarians excellent tanks.
Fighters have the reputation of doing slightly lower damage per attack than barbarians but being much more reliable over time, and Battlemaster is very useful.
Clerics are very awesome. They don’t get the extra melee attacks at level 5 that Barb/Fighter do, but putting them in melee with Spirit Guardians and Spiritual Weapon does a lot of damage. Spirit Guardians may be their best spell - you can walk around the battlefield catching new enemies in it in the same turn. Healing Word as a bonus action to get people up again mid-fight is essential, Cure Wounds is excellent if you can touch the person. Guidance (and Bless if you have the patience to make that your concentration spell) are so, so powerful. Guiding bolt is great as a range attack, inflict wounds is usually much better damage (and chance to hit) than a melee attack with a weapon, since your Wisdom is usually higher than your Str/Dex. In terms of subclasses in BG3, Light Clerics are insanely good - they get fireball and also “flaring ward” which forces enemies to re-roll hits and maybe miss. In 5e Spirit Guardians is one of their biggest spells.
I never understood Warlocks - their eldritch blast (with agonizing upgrade) is very good, but they’re not the best at anything. The Hexblade is really strong in tabletop D&D, the other subclasses much weaker.
Wizards are maybe the most powerful in D&D (especially at higher levels). Some of their best value is from buffing other party members, not doing straight damage.
For example, when a fighter uses their main action to attack after level 5 they get to do two attacks with that one main action. The Haste spell gives you +1 action, so fighter/barb will get 4 attacks if you cast haste on them. By the time they get 3 attacks a round, that goes to 6 with haste. That can be huge. (This is new in 5e/BG3, haste worked differently before).
Essential spells in BG3: Magic missile, cloud of daggers, hold person, misty step, fly, fireball, mirror image for defence. You can ignore most of the others.
Druids are great fun. In 5e their animal forms have their own hitpoints, so when they go to 0hp the druid pops back to their humanoid form with the druid’s original hitpoints still there. Transform again and you get the new form’s hp again. This means that animal forms are free expendable hitpoints and makes Druids very tanky. Note though: it takes a full turn to transform, so you can’t attack in the same round. That means transforming is ALL you do until the next round, which is very slow. Moon Druid subclass get to transform as a bonus action, so can transform and then charge in and attack immediately. So if you want to be melee instead of a caster, go Moon Druid. They’re VERY strong in 5e tabletop.
Druid spells are fun (but quite weak in BG3 before level 7). Spike growth is surprisingly good, thorn whip can pull enemies off high ledges. Guidance, healing word/cure wounds, heat metal, call lightning is probably better than moonbeam as it can be replaced anywhere in later rounds while moonbeam can only move a short distance.
Sorcerers get a very small number of spells but “metamagic” to modify them. You need to know exactly what you’re going to take, because you can’t get more. (Sorcerers were one of the most powerful in BG2 but are very different in 5e, so I don’t know how well they do in BG3).
Paladins are very strong: all the fighter advantages with high charisma and some healing as well, maybe Advantage to hit / more damage with the right subclass.
Okay, I actually have to work now 
