People aged 16 and 17, and children aged 12 to 15 who are eligible, will be contacted by a local NHS service such as a GP surgery to book their vaccination appointments.
Some walk-in COVID-19 vaccination sites are offering the vaccine to people aged 16 and 17. You can check if a site is available near you at https://www.nhs.uk/conditions/coronavirus-covid-19/coronavirus-vaccination/coronavirus-vaccine/
See this post by the BBC for more information about children receiving the vaccine Covid: Which children are being vaccinated and why? – BBC News