TVXQ - Mirotic ^^
Infinite was the first kpop group I was really into
with the first song being Paradise, as far as I remember


GOT7 is the first kpop group where I put fandom effort into getting views, tweet stuff, vote everywhere, listen to endless genie links, and which I bought CDs off 
It's with Just Right that I betrayed Infinite's Bad ^^'

And finally, BTS is the group I'm currently very crazy about, they have so much talent *o*
I need U and Dope were the songs that got me hooked  
It didn't start with one song or one band for me. That was mix of kpop of best kpop songs released by 2011-2012.

But first band I got obsessed with was: CNBLUE
BIG BANG - HARU HARU. ♡
for me it was 4 Minutes song  - Hot Issue

my first K-pop song was "Passion"- by Se7en
my first group would be 2NE1.
I was into Japanese before Korean. I'm not exactly sure how I got into K-pop (it was sort of part of my life before I knew it lol) but I think it was Younha. She sings "Houki Boshi" from Bleach if memory serves me right and I was amazed that she was actually Korean. My sister made me listen to "Password 486" and I totally loved it.

I sort of knew Super Junior/SNSD/DBSK existed, but I never searched them up and stuff. I knew their songs, though (Sorry, Sorry, Gee/Oh, Mirotic lol) I remember SHINee & Big Bang songs, so I think I can say they were my official first? Up until I graduated high school. (Girl groups include 2ne1 and random songs like Chu by f(x) and Abracadabra by BEG.)

College was the start of the over-the-top drama watching and K-pop listening. (Before college, I went to bed like regular people haha) The groups I became obsessed with after SHINee/Big Bang/SuJu died down were EXO (I can't even begin to describe how crazy I was lol) & B2st. Watching all their MVs and shows etc (Other groups like 2PM, B.A.P, CNBlue, F.T.Island, B1A4 just found their way into my life.)

Currently I'm head over heels for BTS. I got a whiff of them back when they released Boys In Luv, and although it was a nice song, it was a bit different to what I was used to so it didn't really hit me until after I Need You and Dope came along and they just stole my heart in a matter of minutes. Now I can take a national exam on BTS and probably score a 90% on a bad day. xD
4minute - Volume Up, still my favorite kpop song till this day. 
The first Kpop group that introduced me to the world of Kpop was Super Junior (and to some extent DBSK), but CNBlue were the first band I actually fell in love with!

I also enjoyed songs by SHINee and Big Bang though.
Big Bang was definitely the one that got me into K-Pop for real; SNSD was the first I head about though.
It was and still is T-ara - Absolute First was an amazing album.
SUPER JUNIOR WITH BONAMANA