Thushyanthan Kandasamy

Hello there, I'm currently working a company as a employee earning around £40,000. I want to set up Amazon FBA account to sell goods on amazon store, after researching i think i will set up as Sole Trader. I know that i need to complete my Self Assessment each year but im not sure on few things: So e.g. if i earn profit of £13,000 on selling Amazon and when to complete Self Assessment: do i need to pay just tax on the £430 (because after minu tax free allowance £12,570) OR do i need to include 40,000 whcih I'm earning as an employee and pay 40% tax because if i add 40,000 + 13,000 = 53,000 ? Could some clarify this for me please?