You will need to register your UK company for Corporation Tax within 3 (three) months of when it begins to trade, i.e. from when it starts any business activity, which includes buying, selling, advertising, renting a property and employing someone, etc.
You can register for corporation tax online and find more information on https://www.gov.uk/limited-company-formation/set-up-your-company-for-corporation-tax. To do this you will need your company’s Government Gateway user ID and password in order to log in.
If you do not have a Government Gateway user ID then you can create one when you go to the website and register your company online. In the course of the registration process you will be asked to provide both your company registration number (CRN) and your Company’s Unique Taxpayer Reference (UTR).