Find out what visa requirements you must meet to enter the United Kingdom by visiting this website:
https://www.gov.uk/check-uk-visa
You will be able to select what country you are coming from, and the site will provide recommendations for next steps.
Electronic Travel Authorisation (ETA)
ETAs are now mandatory for travel to the United Kingdom.
The following link is the best overview, where visitors can find official information and apply:
https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta
Another helpful link, which details key deadlines for groups of visitors (by country), is as follows:
https://www.gov.uk/guidance/check-when-you-can-get-an-electronic-travel-authorisation-eta
Each individual visitor is responsible for ensuring they have their own correct authorisation to enter the United Kingdom.