it's f(x) , Rum Pum Pum Pum
I knew the kpop because of f(x) 
SS501 - Love Ya
they were not the very first kpop group and song i heard. but the first to make such an impression on me that got me crazy about kpop for a long while ^___^
BigBang - Haru Haru 
For me it was SHINee <3 My first song was "Ring ding dong", but i laughted at it, found it so funny and bad. So weird to think about now..... lol... Although i diden't like the song, the dance was cool. So i ended up watching the mv a couple of times. Then i found "Lucifer". I hated the song. So bad, but OMG THE AWSOME DANCING!!! So i kept replaying the mv. For some reason that i don't remember i started watching "SHINee Hello baby". And that was it. LOOOOOOVE for the members ;) kk <3 And as i started to get use to the songs, i found my self falling for them as well ;) 
SHINee Ring Ding Dong.. the dancing was so perfectly in sync and the song was just <333 after watching a couple of their MV's, I watched their reality show Hello Baby and I was hooked! after that I discovered Running Man and the whole new world of Asian dramas xD
@camwicks Lol! I just read your comment after commenting xD we have a similar story haha what a weird coincidence! that's never happened to me before
i knew kpop bcz of missA's Suzy when i saw her in Dream High,
then i search for her then learn about miss A,
and the first song i heard is Bad Girl Good Girl :D 
BIGBANG - Blue
Super Junior - Marry U
TVXQ! It started in... ahm... a bunch of years ago, I was still in middle school (I'm in college now, just for comparison) when I saw a JaeJoong pic in the back of the copybook of a girl from another class... I was fascinated by such beauty, so approached her to ask who he was (my shyness disappeared in that moment, I was hypnotized) and went Google it up later in the same day. Watched Mirotic once and bam, hooked.
First band- Big Bang- Tonight
First song that really made me fall in love with kpop was Fantastic Baby- Big Bang
i've heard a couple of korean songs back then but didn't really know who sang it, 
then i saw on Tv Super Junior singing Don't Don in top 10 music charts in our country
i think that's when i started to listening to their music and became an elf.
I got into kpop world in a very strange way.

On youtube Joe Hisaishi -> Yiruma -> My first drama "Spring Waltz" (a part of the OST is composed/played by Yiruma) -> "Hello Hello" by FT Island.
And after that I'm still crazy about this group and other artists from FNC Ent.
My frist kpop group was SS501 with UR Man in 2008 and that was the beginning of my addiction xD
Girls Generation, 2NE1, Big Bang and Shinee.
Of course. They are one of the biggest, so i get them suggested on Youtube. ^^

But now that changed alot. I like to listen to them, but they are not my favs groups anymore.