I'd advise against getting into k-pop, but if someone still wants to get into it... Fine. But don't join a fandom, or make one your whole identity, or wtv. Fandoms can be fun, but at times they can be cultish and scary. Best not to go too deep into the fandom culture. You can be a fan of someone without being a hive-minded fandom member. That means not getting into fanwars too. Believe me, just because your fave's song came on as an ad and another idol skipped it doesn't mean it's the end of the fucking world.
Don't spend much (or any) money. Especially if the group you like is already very very famous, and very very rich.
Your faves like Justin Bieber. So in continuation of the above two points... Do you really want to invest so much time, energy, and money on a bunch of people who like Justin Bieber? No. You do not. Your faves aren't going to do your homework/degree/chores/work/whatever for you. Spend some time on yourself, and not so much on them.
Speaking of what your faves are doing... Look, this industry isn't all daisies and roses. It certainly isn't like what those xenophobic people who harp on and on about the "dark side of kpop" say, but it's very dark. I've heard enough horror stories, i know. So don't even fucking think about entering it. Just don't.
Oh, and give shinee a listen