There are several routes that you could follow to become a teacher, the advice below relates to a traditional route of A levels > University Degree > Post Graduate teacher training course.
Required qualifications:
You are required to have a GCSE pass at C or above in English and Mathematics.
You'll need a degree to get onto a teacher training course. The content of the degree doesn't have to be the same as the subject that you want to teach - though it makes a lot of sense to ensure that its related. So to be a history teacher things such as History (obviously) archaeology, ancient history, politics and social sciences would be perfectly acceptable. (There are other degrees that would be suitable).
NB: The grade awarded on your degree can influence the rate of pay that you get as a newly qualified teacher.
Post Graduate certificate in education (PGCE). You do this course once you have passed your degree. Typically it is a one year course based partly in university / colleage and partly in schools. The qualification that you receive includes Qualified Teacher Status which then gives you the right to be registered by the general teachers council, the depratment for education etc...
In short, you need:
English and maths GCSE passes.
A levels (or equivalent)
Degree
PGCE (inc QTS)
One of the alternatve routes is to do a 4 year degree that covers an academic area and education. This approach is more normally adopted for primary education.
Whats the course like?
Each training institution will be slightly different in the way that it runs the course so the following is just a general guide.
The college that sends students to the school I work in has one term of predominantly university based learning. Student teachers learn about educational philosophies, the theories behind teaching and learning styles, do a lot of work on what makes an effective lesson / resource etc and get to grips with education law. after a few weeks students then start visiting schools for a day a week to familiarise themselves with the people and systems in place at the school they'll be training at. At some stage there's a week placed in a primary school. Often this is one of the feeder schools of the school that there training will be in. The spring term is then spent doing the 'on the job' training in a school. This involves not only teaching but evaluating lessons, observing teachers and students and doing some traiing exercises to ensure that they can prove competency in all of the areas they are assessed on. There are a few days back in University during this period - which are really just to make sure everything is ok. after 12 weks in one school the student teacher then moves onto a second school. They spend 8 weeks (I think) in this school. The purpose of the second placement is to verify that they can teach and offer an alternative setting in which they can develop their skils.
In both schools the student will have a mentor. Typically this will be the head of department. They will also have a link to a senior teacher who is responsible for staff development and training. At the school I work in the deputy head would meet with all of the student teachers evey week and I would have a debriefing meeting with the student teacher formally once a week and informally after each lesson I observe teaching.
Good points about it?
If you're cut out for the job you will find the course extremely rewarding. Working with children can be demanding at times but there's SO MUCH you can get out of the job.
Bad points?
Any 'bad' points on the course can be overcome through talking to mentors, university tutors and other people on the course. Most problems I've encountered stem from time management problems, easily solved.