Many TAs I have come across are parent helpers with no qualifications who have been employed after volunteering.
Others have done a Teaching assistant level 2 or 3 course at college in either the evening or day and volunteered at a school during this time.
There's also many in house courses once you are in a school, from day courses on subjects to cover supervision which is designed so you are qualified to cover for a sick teacher or planning time for a short period.
There's also a higher level teaching assistant course which attracts higher pay.
Hours and class depend on schools, you may support a specific child with sen Or ability group in one class only, you may split your time between three classes, you may work 8 - 3.30, 9 - 3 or half days or even set days a week.
There are various level 2 or 3 qualifications you can take, usually involving at least one day a week in a school. Most jobs seem to be advertised at level 2, but you can get a level 1 job without any qualifications if you have experience. I started my job at level 1, no TA qualifications but I had experience volunteering in the DC's preschool.
It is a great job, very rewarding and the hours can fit in very nicely with your own DC if you are in the same school or nearby. But be warned, nobody does it for the pay, which is awful. Also, depending on the school, you may be expected to plan your own sessions for working with specific children, or deliver specific interventions with targets- it is a lot more than just a bit of photocopying and hearing children read (which is all some people seem to think it is!) .
The hours etc really vary- be aware that anything from 25 to 30 hours could end up being full time school hours, depending on whether you are paid for breaks/lunch hour. In our school the specific working hours are negotiated with the class teacher, then approved (or not) by the head. Eg I start at 8.15 each day so that I can leave at 3.15 to pick up the DC.
In our school you may move year groups sometimes, but will nearly always stay within either KS1 or KS2.
It was a massive career change for me and the lowest paid job I've ever had, but I love it!