Do I need to open an activity to sell merchandise online (via a third party company)?

[King of Hearts]

Hello,

Finally, I'm starting my own business.  I'm going o start selling mugs and such things on the internet.  But do I need to open an activity or a company to do so? I'm a beginner in this "world" and this legalise dialect seems more confusing than illuminating  

I'll use company already established to outsource the printing and so on... and those companies already take care of the taxes with the customer. (Plus they're international)

All the research I made has not provided me with an answer so far... 

Any help would be greatly appreciated.