Japan being an volcanically active country, has thousands of onsen. 

There is a whole etiquette to keep in mind when bathing in hot springs which can be a little bit intimidating. 

1. Take a small and big towel with you to the hot spring. Some hot springs also need you to bring your own shampoo and rinse but generally this is covered by the hot spring facility.
2. Probably the most important rule to keep in mind is to rinse yourself before entering the hot spring. Modern hot springs have a shower section before the pool where you should take place at one of the low chairs and shampoo and scrub yourself. Older onsen facilities sometimes do not have a shower sections but they do have a bucket. Scoop some water from the hot spring and thoroughly wash yourself.
3. After the first wash/scrub, you can take a seat in the hot spring. Make sure there is no more soap on you before you do. Most people spend 10 to 20 minutes in the pool.
4. Get out of the pool and hit the showers where you’ll repeat the whole showering and scrubbing sequence.
5. Before heading to the dressing room, make sure that you dry yourself as much as possible with the small towel. You’ll hear people grumble if you go to the dressing room and leave wet prints everywhere.

Note

As most hot springs are very hot, it’s not recommended to drink alcohol before going to the hot springs. The hot water will make the alcohol rush to your head which will cause fainting incidents.

Be aware that when visiting an onsen, you will not be allowed to go in with a swinsuit. Hot springs are for full nudity and no Japanese would ever think of it as being anything other than completely normal. No swim suits allowed

More than half of onsen facilities ban tattoos. There is a request from the Japanese Travel Agency to facilities to allow foreigners with tattoos to enter the facility anyways. Japanese with tattoos however, are not shown the same courtesy.

The problem however is not just the owners of the onsen facility. Many families come to hot springs as well and when they come there, seeing a tattooed person in the hot spring will more likely alienate them from the onsen facility than not.