Go on habbohia.com. In the top right corner click ‘Forum’. Alternatively the website link is: forum.habbohia.com.
On the top right corner you will see the ‘Register’.
It will give you a Terms & Conditions Page. Read through and accept. It is important you read through this so you don't make any mistakes while on the forum!
Follow the instructions on the page and create your login. You must NOT use the same password as you use on Habbo. When complete click ‘Submit Registration. Now all you have to do is activate it from your email and you're done!