You'll have to know Japanese to follow through the registration process, or try your luck in guessing ;)
http://www.johnnys-net.jp/j/jweb/jweb_app.html - here's where you need to choose which smartphone you have, iPhone or Android and proceed from there.
Please note that you must have a valid credit card if you're registering from outside of Japan. You'll be charged monthly fee of 315 yen for the usage of the site.
I'll try to write a more detailed info on registration if I'll have time later today, if anyone would be able to help faster - it'd be great :)
crossposted to kanjani8 comm A huge thanks to spurious for tutorial on joining - available here