But for Japan it's not really necessary. Travelling by train is faster and easier. Shinkansen (high speed trains) are reliable and go between major and minor cities. Buying a rail pass before you travel is cheaper than buying in Japan.

Also, you should check your visa requirements. If you are a US citizen and are only travelling for a holiday, you are exempt and don't need a visa. You should check the validity of your passport. There may be a minimum requirement of time remaining. However, if you've never travelled, you may need to get one.

Renting a car would not be a good idea. As others mentioned, public transit is excellent so you'd be making it harder in many ways (parking, traffic) but also driving in another country can be so unexpectedly different in minor but important ways. Different rules of the road, and driving on the left side just reverses so many instincts in where to look in the road.

I've traveled around a lot. The only times I've really been glad to have a car outside the US was in Oman (public transit is terrible) and to a much lesser extent, Norway (visiting family in smaller towns)

I know a lot of people are saying don't rent a car, but if you like to drive, the roads in Japan are great, the rules are followed, it's very relaxing place to drive. And a great way to see the countryside.

So I 100% recommend you get a car and drive around if you like driving around and exploring things. Just avoid the big cities. Any of the second or third tier cities should be fine to drive out of, and the countryside you absolutely need a car.