Confused about tax

Hi,

I run an online shop selling phones, mainly cordless phone systems. I'm
wondering about how income tax works, as the shop is starting to make
more sales so I'm going to be developing it more. My question is how
does income tax work on selling items? Looking at things like this
'online tax calculator' (https://www.income-tax.co.uk/), they say I
should start paying tax if I include the money from the shop (as I only
work part time). What do I need to do about this? Thanks in advance!

Julien




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JulienI