The best way to get a picture of what's what will be to google 'children's tuition (your postcode)' and see what comes up. Different areas have different requirements. In areas with aspirant immigrants, extra English tuition plus extra tuition in curricular subjects is a steady area of demand, but you can expect to be bartered with for 'your best price', or asked to attend at convenient times like Sunday mornings. Posh suburbs often yield families who want a child crammed for private school entrance exams. Oddly enough families who send their kids to private schools are often also keen on extra tuition - all that money spent on the schooling isn't apparently guaranteeing a good experience for all!
Rather than setting up all alone why not try your luck with one of the many franchises already around, like Kumon or Tutordoctor.
I think you will find that teaching qualifications and experience are expected.