[Nao]

You need to make sure you have a way to access your money. If you're planning on using credit or debit/checking cards, not all foreign-issue cards will work in all places. Post offices will generally allow foreign check/credit cards at their ATM though, so always keep that in mind. Japan is largely a cash society - using a card for many purchases is not overly common (though many people do seem to use their Suica card and the like for purchases in many convenience stores).

Do not worry too much about carrying fairly large sums of money. It's perfectly normal to be carrying ￥20,000 - ￥50,000 at a time.

Coins will build up if you don't use them frequently. Coin denominations are as follows: 1, 5, 10, 50, 100, 500. Bills: 1,000, 2,000(rare), 5,000, 10,000.

I also recommend either exchanging currencies in some amount before you disembark from home, or have some USD or the like to exchange at the counter after you land. You will need cash as soon as you get there, likely even before you get somewhere that you can withdraw it. Manage your money carefully.

Next, make sure you are at least somewhat familiar with the rail system, and plan especially how you are going to get from the airport to where you're staying, and back. Review maps of your area and see how the rail lines connect with major lines, or if you're at a station on a major line. Also make note of nearby post offices, and any other place you may need to be aware of beforehand.

Aside from these two obvious points, you need to be a little less vague and give us some information regarding your itinerary and plans if you want help -- as well as an idea of what you need to know.

-Nao