These reloadable IC cards work on every Tokyo train, bus, taxi, vending machine, and most konbini. Buy one on day one.
Welcome Suica (tourist version) doesn't require a deposit and expires in 28 days. Regular Suica requires a 500 yen deposit, refundable.
Load it with 3,000–5,000 yen to start. Top up at any station machine or konbini.