With spells such as haste, the description talks about how a 'hasted creature gains a -2 initiative bonus', so 'a creature moving at 6 and attacking once per round would move at 12 and attack twice per round.'

My question is, how can you find out your initiative, and what about these numbers '6 and 12?' I never really understood them, and until reading through the spells again, I thought that the higher your initiative, the better. But now I see that it says you gain a -2 initiative bonus, so how does it all work?

This is AD&D core rules. Unsure what if anything is changed in BG2.

In any given combat, all of the participants have to roll an X sided die (it says 6 in the rulebook, but I believe it's different in BG.) Lowest roll goes first move. Next lowest gets second, and so on and so forth.

Initiative itself is a random number. However, with spells (like haste) and other factors, you can floor it. I don't believe the -2 modifier has anything to do with the other part though: they're separate parts of the same spell.