Good advice above from Carol NR. In addition...
BTW, the plural of yen, the Japanese currency, is just yen (with no trailing 's')...so it's "one yen, or a hundred yen" not "yens".
It's quick, easy, simple, convenient, and cheap to get cash at an ATM by withdrawing cash from your checking or savings account at home - same as in Europe or in your home town. There's really no need to "buy" local currency in advance, no need to "change money". Many ATMs are waiting in the arrivals hall at the airport (and as Carol said, you can pay with a credit card almost everywhere). I've been to Japan many times and never bothered to exchange money or "buy" yen before I arrived, just hit an ATM upon arrival, or when you need more. As stated previously, credit cards are widely accepted, same as at home. You might need cash occasionally for some small purchases (an ice cream, or automated vending machines), but generally money is easy there.
I want to stress how widespread the (what we would call) "convenience stores" are: 7-11, Family Mart, and Lawson (similar) are everywhere. In some areas, there will be several to choose from on a single block. You can also get cash from ATMs in most Japanese post offices. All the ATMs (and most any vending machine) will have an option to choose English for the interface language. It's easy.
Don't use a credit card to get cash (in Japan or anywhere) unless it's a last-resort emergency - pulling cash from a credit card will cost you a lot in interest (which starts right away). Use your debit/ATM card from home for cash, and use your credit card (with no foreign transaction fees) to pay for anything other than getting cash.
It's easy to feel a little intimidated before your first trip to Japan, because from the outside it appears so "foreign" and exotic. Don't worry, it's much easier to function there than you think it will be - in general, things in Japan all "just work" and it's set up well for clueless foreign visitors - and people are generally helpful and nice. really, it's a very easy country to travel in. If you can manage London or Paris, you can manage Tokyo. Really, you'll do fine, you will soon see - so don't worry.
Wear whatever you want, nobody will care. As a tourist, they expect you to be kind of clueless and maybe a little weird. Just be polite, and they'll treat you right. Japan is a hoot. Have fun.