Income could be non-savings i.e. salary, trading profit, rental income etc, or savings i.e. interest income from bank and financial institutions or dividend i.e. dividend from UK or foreign companies.
If you, as an individual, have salary or interest income, employer or the bank and financial institution deduct tax at source. If you have trading profit or rental income or any other income, you need to file your personal tax return. In order to do that, you have to register with HMRC.
You need to notify HMRC about the start of your trade or rental business by 5th October after the end of fiscal year which runs from 6th April to 5th April.
You have to submit paper tax return by 31st October or online return by 31st January after the fiscal year. If you fail to submit so, there is penalty and interest.