Depends if your employed or self-employed.
If you are employed you will be taxed on the PAYE or Pay-as-you-earn scheme.
Whereas, when you're self-employed you have to keep your own accounts and you can offset certain purchases gaainst income to create a loss. If you make a loss you don't pay any tax ...but you should still pay the minimum National Insurance contributions.
In the UK you get a tax free allowance of £5,035 That means that you can earn up to that amount before paying any tax.
However, there is an assumption made that you will earn more than that and so the government doesn't wait until you reach that amount before deducting tax. It takes off an agreed percentage each week or month.
If you don't earn more than £5,035 you can reclaim the tax paid or have your tax code adjusted for the next tax year (April 6th to April 5th)