To succeed as a writer you do not necessarily need qualifications but will need to be able to develop ideas that will sell, have good research skills and be able to express ideas in a style suited to your intended audience.
For book writing you can download a factsheet on getting work published from the Booktrust website - this includes a list of books about writing and getting published.
For fiction, most publishers will only consider noncommissioned work through an agent - lists of agents and information on submitting your work are in The Writers' Handbook and The Writers' and Artists' Yearbook.
For other types of writing you could approach publishers direct - it is important to research which publishers might be interested in your work.
You will usually submit one or two sample chapters and an outline of the complete work and will likely wait several months for a reply - be prepared for rejection or not receiving a reply.
For radio/TV you can find information on writing and submitting scripts by visiting the BBC Writersroom website. Competition is strong for TV work - it will be useful to have a track record in another area, e.g. having a radio or stage play performed, but it can be difficult to get producers or broadcasters to look at your work if you do not have an agent.
See The Writers' and Artists' Year Book for details of agents who will consider unsolicited material.