How to become a VIP on Habbo for Free?

Simple Answer, you can't.

The only people that have access to VIP clothing are (obviously) VIP's. In order to become a VIP you must pay the specified amount of Habbo credits and you will only have access to the VIP clothing WHILST you are a VIP.

There is an option when changing your looks to try on Habbo Club clothes (which also allows you to try on VIP clothes) but you cannot save yourself wearing these clothes. These clothes are only for view, in order to wear them you must be a VIP.