well..it really just depends. i've been to all the music shows a few times but the only one i haven't gone to was inkigayo. you do usually need tickets to each one, but foreigners can't really do that. if you don't have a ticket, you can line up with the fan club. each music show is different though, and each fan club. one club might require nothing, but others will require a membership id (big bang's vips actually have id cards with their picture on it that they have to bring), or registration online with the fan cafe (like 2ne1). also you do usually need the singer's cd with you.
mm..i know music bank changed their rules a while ago, so that only ppl with tickets already can go in. so it really sucks. however, all you need to do is call the reservation number to reserve tickets (i believe you can reserve up to 2 each phone number). you can start calling in after each music bank performance until tickets are gone.
for people lining up really early, it really depends on the singer. like for shin hyesung's comeback..my friend and i wanted to wait to see him..and came in the balsan area around 10 am of that day for inkigayo (they start letting in around 2?) by the time we came..we were numbered in the 400s? but it was just for hyesung. people did camp outside overnight to watch him. however, other singers had less, but we didn't have anyone else's cd with us so we just waited. well..they ended up only letting in about 200 of us around 2? but by that time there was like..800 waiting just for him (maybe more i dunno)...vs at that time there were other fans maybe 50ish? waiting for their singer. for all the shows i think the codis (in charge of the fan club that day or wahtever) are randomly assigned who goes first and wahtnot. so if you have a lot of fans and you go first basically all of you get in..so it depends.
mm...mnet countdown just changed the main location of the show to nutrikrum square in digital media city (it was actually down the street from where i was staying). before, the location was bigger and people could sit and stuff. at this place..everyone squishes in and stands like..in a block. it's really hot and sweaty. think of a club kind of performance with screaming high school girls. not pretty. for mnet countdown you can't reserve tickets (unless it's a special broadcasting or something), so you should check with the fanclub that you want to go in with's cafe to see what the rules are.
music core is the most out of the way of the shows..it's about an 1 and a half from seoul (bc it's in ilsan). there's a great mall next to it by the way! it's pretty big. fans can reserve online for tickets and fan clubs do wait. of course people with tickets get in first. i would say the music core's security is the nicest out of all of them and lets the most fans in. (they pack it in: fans sit on the stairs and stand in the back, it's pretty crazy).
for all of them..i would say talk to the coordinator of the fan club first when you get there, and then you can go off and do your own thing for a while. (they write a number on your hand, and take down your name and phone num just in case you aren't there when they start lining up by number.) come back around the time they start letting people in and get in line according to your number.
also, they're really strict on cameras and stuff. if they see you take out your phone, they'll also make you put it away. after one warning, they'll kick you out of the show. also! usually when you go to a music show, you usually receive a present of some sort, like an autographed picture of the singer or something.
just let me know if you have any questions. if you want i can describe to you how to get to each show and what's around it to do.
by the way it would be better to have someone who can speak korean (for communication purposes). i'm not korean, but luckily i do speak a little of it so it was definitely easier.

. also, don't have really high expectations before going to these shows, because seeing them live is a lot different than you think.