As wee-janine says, the job can vary greatly. In Primary school you may be attached to a form teacher and therefore get involved in the classroom administration and preparing lesson resources as well as working with individual or groups of children as required by the teacher. Or you may work across the school, perhaps taking groups of children for maths and literacy, again under instructions from teachers. Some Teaching Assistants will work only with one child who has special needs.
Check out your local FE college - there are a number of different courses you can do, but you will still need to get some experience in a school setting.
One key area that you will need to be able to discuss in an interview is behaviour management.