As an au pair you do not require any special qualifications (as a nanny would) or any special psychological training. As an au pair your duties will include taking care of the children, perhaps a pet, doing light housework such as vacuuming and dusting, as well as helping with preparation of meals. You will usually have to pay for travel costs to get to your host family and any necessary language classes yourself. Typically you will work 30 to 35 hours a week, probably with some additional babysitting a couple of nights a week too.
Regarding visas, if you are a citizen of Australia, New Zealand, Canada, France, Korea, Germany or the UK and are currently living in your country of citizenship you may apply for a Working holiday visa. In order to be eligible your plan must be to visit Japan primarily for pleasure, and for a specific length of time. You must be between 18 and 30 and shouldn’t have a criminal record or any serious illness. Check with the Japanese Embassy or consulate where you are for more details.
In exchange for your time your host family should:
There are several very comprehensive websites that can help you if you're interested in doing au pair work in Tokyo: