Countdown Live's tickets (for ayu at least, not sure about other artists) are offered to Teamayu members through a lottery. Normally, many TA members who are friends enter and get more chances to get tickets, although it's very hard.
The easiest way if you aren't a TA member is to get the tickets from a reseller shop or as Suteisi said, through a middle-man.
Not sure if you can actually line up on the day the tickets go on sale, it doesn't work like that in Japan, correct me if I'm wrong
