You'll need a good honours degree in a subject relevant to the national curriculum. You'll also need maths, english and science at GCSE grades A-C, and a few A levels wouldn't come amiss either (most people will have these by the time they get to university in any case). On top of that. you'll need to have completed a one year Postgraduate Certificate in Education (PGCE).
You can also go in via a four year course that includes degree and QTS (qualified teacher status), or there are courses where you can, so to speak, learn on the job.
Whatever route you choose, you will need to have that degree and postgraduate qualification before you can start teaching 'proper'.