Energy around the world can be confusing if you never left the homogeneous energy supply of your home country. There are different voltages (V), different frequencies (Hz) and different power plug types, all to the ultimate confusion to the unaware traveler. The image below should suffice for the plug type:
The voltage in Japan is an homogeneous 100V (will work with 110V rated devices). This means that it is on the lower end of current and it should not instantly burn appliances that are 220V accidentally plugged in the wall, but if you insist on trying to use an appliance on the wrong voltage (if it even works), it will eventually damage it or toast it, so don't. Auto-switch devices are safe to use everywhere, and are usually stamped 100V~230V (some older are 110V~220V but will work too)
The AC frequency changes between 50Hz and 60Hz: 60hz for west of Tokyo, and 50Hz for Tokyo and east/north. Most modern devices will auto-switch but you should also take notice. Auto-switch devices will read 50Hz~60Hz.
Do not try to use a device that is not compatible with 100V and the frequency of the region, while some types of device might even turn on, you are running a very high risk of damage to the device or personal electrical shock
If your devices are compatible, you might need only a plug adapter. There are some cheap plug adapters on most electronics/hardware stores, but stay away from the ones sold at Airports: they are always overpriced. If you need a voltage conversor, you might consider either purchasing a more modern device that have auto-switch, or look for the simpler voltage conversor possible (remember, input should be 100V~110V, output whatever you need) - do note they are usually kind of heavy so purchasing a new tool is usually more convenient.