When you start working for yourself, you do not get your first tax bill for a while, so it may help your budgeting to start putting money aside now.
The ready reckoner tool can help you with this. It shows you roughly how much money you might need to meet your Income Tax and Class 4 National Insurance bill. It doesn't take into consideration any Payments on Account you may have to make.
You will need to pay your Class 2 National Insurance contributions separately.
The ready reckoner tool is for self-employed people who have no other taxable income and are entitled to a basic Personal Allowance. You fill in your estimated weekly or monthly profit. Your profit is your income less any business expenses. It is assumed that you will make your accounts up to the 5th April each year.
You only know the exact amount of Income Tax and/or Class 4 National Insurance contributions you owe once you have completed your Self Assessment tax return.