There are some useful starting points here:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/profiles/profile820/
Your first step should be to get yourself onto a course to gain a GCSE (A to C) grade in Maths and in at least one science subject. Although that link I've provided states 'Alternative qualifications may be accepted' that only relates to qualifications which are of at least the same standard as GCSE (such as certain overseas qualifications). Basically, if you can't get a grade A to C pass in maths, you can't become a teacher. (As a qualified maths teacher, at secondary level, who spent many years trying to undo the damage done by certain poorly-qualified primary teachers, I can only applaud that decision).
You'll need a university degree to qualify as a teacher. While it's possible that you might be accepted, as a mature student, onto a course without any A-levels, you'd certainly improve your chances by passing at least one A-level subject. (As with the GCSE mathematics and science, that probably means a year or two of evening classes).
If you want to be able to impress the people who'll giving consideration to your application for a place on a course, it would be wise to do a bit of private study in advance. (Read at least one book each on educational philosophy and developmental psychology, so that you can prove your commitment to study).
Also see here:
http://www.tda.gov.uk/Recruit.aspx
Chris