Teachers are divided into two main categories: those who become primary school teachers and those who choose to teach in the secondary school environment. If you are looking to become a teacher you will need to decide from the onset which way you want to go, and make your education choices based on that choice.
Primary school teachers have to be prepared to teach all areas of the curriculum, and will normal be assigned to one-year group for the whole year. To become a primary school teacher you will either need to do a specific primary teaching degree, or do a subject based degree of your choice and then take a Post Graduate Certificate of Education (PGCE) in Primary Education the year after you graduate.
Secondary school teachers will be subject specific, and during the course of their working week will see all year groups, in various sets or classes, and will teach only their subject of choice, for example French. To qualify as a secondary school teacher you will either need to take a degree in your chosen field and then a Secondary PGCE, or take a specific degree in education which will be interspersed with the subject matter you wish to teach.
There is not really any chance to change between primary and secondary education, so you need to make sure you are sure you know which you prefer, and you will need good GCSE and A-level results to get you into your chosen university.