In the UK if you want to look after young children you have to register. This is done through the education standards body, Ofsted. You also have to meet the requirements of the early years foundation stage (EYFS), which means that you are able to help the children with basic early education and development needs like language, social skills etc, as well as of course being a safe and competent person to look after them.
If you want to register you should contact your nearest local authority (if you put 'local authority' plus your town name in a search engine you can find the contact information you need) and ask them about registering as a child minder. They can then advise you about where the next pre-registration session is held in your area. If you attend this you will be assessed for suitability, given information packages and generally helped to proceed to the next step. You can find more information here.