Most nationalities can enter Japan visa-free for a short term stay. A short term stay is up to 90 days for tourism, business, visiting friends or relatives etc that does not include remunerative activities.
There are currently 68 countries and regions for visa exemptions including USA, Canada, Australia, New Zealand and the UK. However, please make sure you always check your visa conditions prior to booking a flight, even if you are from a visa exemption countries/regions. You can read more about how to obtain a visa on the official Japanese Ministry of Foreign Affairs.
Safety in Tokyo
Tokyo is a very safe city, even at night. Solo women travellers and group travellers generally find Japan and Tokyo safe. Most visits are trouble-free but as always, it's very important that you purchase travel insurance before you visit Japan.
As with any destination, you should maintain the same level of vigilance and practice common sense. It's safe to walk at night and to take public transport.
Tokyo's entertainment districts are considered at higher risk for crime. Avoid entering clubs and bars in the Kabukicho area without a guide or a local who you can trust. Take steps to ensure your drink cannot be tampered with.