You should think of getting this book called The Little Tokyo Subway Guide. I just got it in the mail and looked through it. The book is pocket sized but not too small and it's packed with info. You can get it for slightly less than $10 on Amazon and it's a worthy investment. I'll be going to Japan in late-May so I bought this book and I'm glad I did. You can read about this book online and see if you want to buy it. It not only has info on all of the 13 subway lines in Tokyo, but also info on ticketing, getting to and from Narita, landmark finder, exit finder on where to go when you get off the train, full-color maps of the entire Tokyo system as well as lines that connect outside of Tokyo to Yokohoma or other areas, useful words and phrases, etc. It's jam-packed with info that was compiled under the supervision of Tokyo Metro Co., & Tokyo Metropolitan Bureau of Transportation. This book was even printed in Japan and is of high quality. It's a worthy investment for your trip. You should look into it.
Amazon.com: The Little Tokyo Subway Guidebook: Everything You Need to Know to Get Around the City and Beyond: IBC Publishing: Books
The $30+ each way to and from Narita seems like one of the pricier fares. You can ride the Keisei Express train for cheaper and it will still get you to your destination nearly around the same time as some of the more expensive options so paying the extra cash doesn't seem to be worth it. Some of the more expensive options can even take longer than the cheap Keisei Express train.
You should open a credit card just so you have some credit history in the U.S. Credit cards are only a bad thing if you're a spendaholic and don't know how to manage your money. Get a credit card that doesn't have an annual fee. Some BoA credit cards have small annual fees so try to get a credit card that doesn't have annual fees like from CitiBank or others. If you want a credit card with BoA then ask them if they have an credit cards without annual fees. If you're responsible with your spending habits then a credit card is nothing to fear. It's only dangerous in the hands of irresponsible individuals.
Make sure there's plenty of money on your debit card and you should search for ATM machines at the post office from what I've read since they accept foregin cards while other ATM machines in Tokyo do not. I read that you're charged $5 for each Bank of America debit card withdrawal so you should just take out a lot of cash at once to last you a while so you don't have to keep paying the $5 fee withdrawing small amounts every time. I also have a BoA debit card that I will be using. Be aware that banks can also freeze your account if you withdraw your money in a foreign country. You could call you bank and tell them that you'll be in Japan at this time so they don't freeze your account thinking that maybe your card was stolen and used overseas. This is what I read when doing my research. I think you should also bring some cash with you on top of your debit card just in case.