If you are in Korea, you need to use Naver and to sign up is pretty easy. All you have to do is go here:

https://nid.naver.com/user2/joinGlobal.nhn?m=init&lang=en_US

And sign up.

Then you just have to sign up for a cafe by clicking on this below. Follow the instructions in the pictures below and you can easily sign up.

how to sign up for a cafe

then just follow this.