Tee hee, you're not the first student to turn up to the wrong classes and I'm sure you won't be the last! (I've had some in mine who suddenly realise they're in the wrong place when I point out I don't have them registered)
Congrats on venturing down the postgrad route
I started off in accounting before turning to linguistics and, nearly 8 years later, I'm getting ready to submit my PhD! When you find something you like and enjoy, the 'study' aspect is so much easier and didn't ever start a new course (at both undergrad and during my MA) without feeling daunted, you just settle in, make friends, build study groups and everything sorts itself out
Good Luck!