Climbing Mount Fuji

Wikivoyage will tell you all you need to know: Mount Fuji

The TL;DR version:

• The mountain is officially open to climbers only from July to early September. Visiting outside the official season is legal but inadvisable unless you know what you're doing, since everything is closed and the weather can be extreme.
• It's cold up there, temps at the summit can be sub-zero even in midsummer. Dress accordingly, and bring wind & rainproof clothing.
• The climb requires no technical skills. It is, however, a long, tedious slog, and your legs will kill you if you're out of shape.
• Most people start at the Kawaguchiko 5th station (bus terminal). There are rest areas ("stations") every couple of km, with hot food and places to sleep, naturally at a steep price. The top is the 10th station, complete with restaurants, post office, vending machines and thousands of other climbers -- glorious wilderness it is not. The other routes are less popular and have less facilities.

Good luck! As the Japanese say, a wise man climbs Fuji once, but only a fool twice.

In addition to jpatokal's answer you have to understand that getting up to Mount Fuji is not really to be considered to be "climbing" or even "trekking" in the real sense of the word. Climbing up there is done by some many people of all ages and there is so much support offered along the way that it's not a challenge in any way.

The issue is that the mountain is so overrun at peak times that it feels more like standing in line for a popular sushi restaurant on a steep slope.

If you are up to something more challenging at the mountain, I recommend this one.

All you need are:

• A flashlight
• Some warm clothing
• Some snacks (there are lots of little shops along the way up, but they all sell stuff at quite exorbitant prices)
• Perhaps a sleeping bag if you don't think you can do it all in one go and think you'll need at least a nap.

It is not technically challenging at all and isn't really mountain 'climbing'; you basically just walk up all the way. It is possibly the easiest 3000m+ mountain to 'climb' in the world.

Most people start from the so-called level 5. The one time I climbed it (July 2007, took me late afternoon to dawn) was from the very bottom --- before reaching level 5, I saw exactly zero other people. But even after, it wasn't crowded at all.

I believe what the Japanese say is that Mount Fuji is good to look at from afar, but not so pretty or worthwhile climbing. I tend to agree. After level 5 especially it's just a bunch of somewhat yucky and unattractive volcanic ash that you'll be trekking through. And when you finally reach the top, there's a good chance everything will be obscured by a thick fog. (When I reached the top, visibility was maybe 1 metre.)

Two things that were not mentioned, and I think would be worth considering:

1. If you don't hike regularly at high altitudes, you may experience severe difficulty at the last stretch (I know I did). Either start early in the day to make sure you have an extra couple of hours to stop and rest, or alternatively, sleep in one of the shelters along the way.

2. The way down is a bit steep, and quite long, especially if you had altitude trouble on the way up - your knees may not be as sturdy as you're used to them being. Make sure you bring at least one trekking pole, or at the very least a sturdy branch.