Self-employed business owners (sole trades & partnerships) pay income tax and national insurance on their taxable profits.
Income tax is charged on your profits less your personal tax allowance (11.5k 17/18).
You will pay 20% on profits up to 33.5k, 40% on profits over 33.5k and less then 150k, anything over 150k will be charged at 45%.
As self-employed you will pay class 4 national insurance, you will pay 9% on profits between 8.1k and 45k, profits above 45k will be charged at 2%.