You don't have to be certified, but you should be mature. Also where I live, one person is only allowed to have 3 children in their care at a time. It would be very helpful to know basic first aid. The parents are trusting you with their child(ren), so you should show them that you are mature and responsible.
No you dont need to be, me personally before i started babysitting i went through red cross training so i'd know what to do in the worst situations. You never know what you'll be faced with so it's best to be prepared.
nope. just have to be responcible. and trust worthy. well you dont HAVE to but it helps. when trying to get a job babysitting
depends on the country