Do search on the forum. Similar questions have been asked many times.
Make sure to keep all the records as you go along.
Sales - Purchases - Expenses associated running the business (e.g phone/internet bills,
postage, stationery and etc) then deduct a personal allowance to get taxable income.
In your example, £4000(sales) less £2000(purchases) less expenses less personal allowance = taxable income. Rather simplified but you get the picture.
Read
here about National Insurance Contributions - you might wanna spend sometime reading around there.
It's a domestic accommodation. You haven't made any strucural alterations to the room in order to run the business so you will not pay business rates.
I would start off as a sold trader and see how it goes.