Opening an online shop: legal issues

Dear everyone,
I am thinking of re-opening an online store I used to have. It always was something small on the side, but I thought I might want to take it further this time. Does anyone know the details on how to do it legally (aka: would I have to be registered as an independent worker? - I heard it's insanely expensive in Belgium- ; what kind of taxes would I have to pay, etc.)? Otherwise, some tips on where to ask (possibly in English!) would be really appreciated! Thanks!