I can probably get over the graphics, and I understand THACO, it's the multiclass vs dual class on top of there being some confusion about how all that changes between BG 1 and 2 and where exactly BG EE fits into all that.
My advice would be ignore dual-class. Dual classing is only available to humans - it's more or less like multiclassing in 5e (yeah, the naming is confusing), but with many restrictions:
Humans may not MULTI-class, but can instead DUAL-class, starting from a Level 2 kitted or a unkitted class onward. By doing so, they cease to advance in their current class and start to develop a new, unkitted class from its Level 1. Once they have changed to a second class, they cannot go back to the first. In addition, the abilities of the first class become unavailable until the second class reaches one level beyond the level where the first class stopped. This option may only ever be done once, even if the requirements are met for more than one new class.
IMPORTANT: The experience cap in Throne of Bhaal is 8,000,000, experience points from both the exact XP requirement of the level of the first class and the total XP from the second class count towards this total. Please make sure that there're sufficient points left available to progress high enough in the second class.
(From the wikia)
I wouldn't touch dual until you know what you're doing or look up a guide. (Personally I just dislike dual-classing, so I might be biased.)
Multiclassing is basically being two classes at the same time - you split XP between them. Can't really go wrong with multiclass, though some are easier to use effectively than others. I'd also suggest not picking pure thief, I found I only needed "half" of a thief in my recent playthroughs - but if you do need more thieving power, there's a lot of thief companions in BG1 to choose from. (I usually go Fighter/Thief for my PC.)
Or you could always go with pure class - multi/dual is strong in BG1/2, but you might want to go pure caster for accessing higher level spells earlier.
Just mess around with character creator.
There's also a neat table on the wikia.As for changes between BG1 and 2 - 2 introduces some 3e content/rules, though I think it was only the case in the classic versions and EE allows you to use all the character options in BG1 as well. (I'm playing classic versions, so take it with a grain of salt.)