Not all businesses need to be registered – it really depends, ie you could operate as a sole trader (just yourself) or set up a Limited Liability Company. Setting up the later is super easy – just do it at the Companies Office and you can do it online. Costs are not massive, but not insignificant – I would guess at less than $1k (I could be out of date).

To build up a customer group, you need word of mouth of people who like what you are about and what you do. To build up a client roster, you need to get clients who love you and tell others about it. Not harder than that.

Good luck!